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Rats responded on a two‐component chain schedule in which a response‐contingent electric shock at the end of the first component was either positively correlated, negatively correlated, or uncorrelated with reinforcement availability in the second component. With 0.4‐ma shocks, rate in the first component depended on the shock‐reinforcement correlation: when shock and reinforcement availability were positively correlated, after extended exposure to the contingencies, rates exceeded those in the absence of shock; when shock and reinforcement availability were negatively correlated, responding was generally suppressed throughout. The discriminative control of shock over responding in the second component, in which reinforcement was available 50% of the time, also depended somewhat on correlation. However, rate change in the first component was not specifically related to discrimination in the second component. With 0.8‐ma shocks, responding was substantially suppressed in the first component at all three values of shock‐reinforcement correlations.

In Exp. I three pigeons were trained on a two‐component chain schedule. Responding on a 1‐min variable‐interval schedule in the initial component led to a sequence of two fixed‐interval schedules in the terminal component. The rate of reinforcement in the terminal component was kept constant while the values of the two fixed intervals were varied. Three combinations of fixed‐interval schedules were studied, FI 0.25, FI 1.75 (minutes) or FI 1.00, FI 1.00, or FI 1.75, FI 0.25. The rate for each subject declined in the initial component as the value of the first fixed interval was increased. Experiment II was conducted to assess the role of the second fixed‐interval schedule in the terminal component in determining the rate of responding in the initial component. For each chain schedule the rate of responding in the initial component was determined both with and without the second of the sequence of fixed intervals. In all three cases the rate of responding in the initial component decreased when the second fixed interval was removed. Increasing the first fixed interval in Exp. I had a greater effect on variable‐interval performance than did the removal of the second fixed interval in Exp. II.

Consider experimental treatments with consequences so irreversible that baseline performance cannot be recovered. The conventional method of assessing the effects of such treatments by statistical means involves separate experimental and control groups. An alternative proposed here is to administer the experimental treatment to each subject, one subject at a time and in a random order; whenever any subject receives the experimental treatment, those subjects which have not yet received it receive a control treatment. This procedure permits results significant at the one‐tailed 0.05 level to be obtained with four subjects; if a two‐group procedure evaluated by means of the U test is used, a minimum of six subjects is needed for the same significance level. More generally, the procedure permits equal sensitivity to any experimental effect with over 30% fewer subjects than a two‐group procedure. Extensions of the basic method are made to a variety of levels of the experimental treatment and to treatments without irreversible effects, and limitations of the method are discussed.

Pigeons were rewarded on a variable time interval for pecking a translucent key illuminated with either a 45° or a vertical line. The key illumination changed every 2 min during daily 1‐hr sessions. When the rates of pecking were stable, reinforcement was omitted in the presence of the 45° line. Responding in the presence of the vertical line increased. This increase did not disappear when responses to the 45° line were once more reinforced, but when reinforcements for responses in the presence of the 45° line were again omitted, responding to the vertical line increased again. After the second alternation of these two procedures, the increased responding to the vertical line appeared when responses were not reinforced in the presence of the 45° line, and disappeared when reinforcement was available during both stimuli. In a second experiment, the key illumination changed between sessions only, so that 1‐hr sessions of reinforcement and non‐reinforcement occurred on alternate days. Responding to the vertical line still increased when responding to the 45° line was not reinforced, but the increase tended to disappear during the session.

Second‐order schedules consisting of sequences of component differential‐reinforcement‐of‐low‐rate schedules were used to investigate two different methods of introducing exteroceptive stimuli. When different exteroceptive stimuli were associated with each component (chained schedule), periods of pausing and lower response rates developed in the early components compared to the early components of the same sequence in the presence of a single exteroceptive stimulus (tandem schedule). When a brief stimulus change occurred at the completion of each component, response rates were higher in the early components compared to the tandem schedule. Changes in response rates in the early components were directly related to changes in reinforcement frequency in the terminal components produced by the two methods of presenting the exteroceptive stimuli.

Two chimpanzees were required to choose between a fixed‐ratio schedule and a progressive‐ratio schedule which increased in response requirement by 20 responses each time it was chosen. Each choice of the fixed ratio reset the progressive ratio to its minimum value. The fixed‐ratio requirement was varied from 40 to 1000 responses. The subjects' preferences for the progressive‐ratio schedule varied as a function of the magnitude of the fixed‐ratio requirement. An analysis of the preference data indicated that the animals tended to minimize reinforcement cost rather than match the progressive‐ratio requirement to the fixed‐ratio requirement. In a second experiment, selection of the fixed ratio did not reset the progressive‐ratio requirement to its minimum value. In this case, the animals matched the progressive‐ratio requirement to the fixed‐ratio requirement. A model based on reinforcement cost is presented which permits accurate prediction of preferences between fixed and progressively increasing ratio schedules.

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