The literature was examined to determine how well the generalized matching law (Baum, 1974) describes multiple-schedule responding. In general, it describes the data well, accounting for a median of 91% of the variance. The median size of the undermatching parameter was 0.46; the median bias parameter was 1.00. The size of the undermatching parameter, and the proportion of the variance accounted for by the equation, varied inversely with the number of schedules conducted, with the number of sessions conducted per schedule, and with the time within a component. The undermatching parameter also varied with the operanda used to produce reinforcers and with the reinforcer used. The undermatching parameter did not vary consistently with component duration or with several other variables. Bias was greater when fewer rather than more schedules were conducted, when two rather than one operanda were used, and when White Carneaux rather than homing pigeons served as subjects. These results imply that the generalized matching law may describe both concurrent and multiple-schedule responding, but that the same variables do not always influence the bias and undermatching parameters in the same way for the two types of schedules.

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In a test of Herrnstein's (1970Herrnstein's ( , 1974 equation for simple schedules, 15 pigeons pecked a key that produced food delivered according to variable-interval schedules. One group of birds was water deprived, and food-reinforced key pecking occurred in the presence of free water. Two other groups were not water deprived; water was present for one and absent for the other. As predicted by Herrnstein, the parameter r. was significantly higher in the water-deprived group than in the two nondeprived groups. Contrary to Herrnstein's interpretation of r., the rate of drinking varied across schedules. Herrnstein's interpretation can be salvaged by considering r0 to be an average. However, if r. is an average, the equation is not a good explanation of behavior because this average is not valid until all schedules have been sampled. In addition, low percentages of variance accounted for suggest that Herrnstein's equation may be of limited usefulness even as a descriptive model for these situations.

Two experiments examined the effects of baseline reinforcement rate and component duration on behavioral contrast and on re-allocation of interim behavior in rats. Positive behavioral contrast occurred during multiple variable-interval 10-second extinction (VI 10 EXT) after a multiple VI 10 VI 10 baseline condition, but not during multiple VI 60 EXT following multiple VI 60 VI 60 baseline. Component duration had no significant effect on contrast. These results differed from those found in studies of pigeons' key pecking. Contrast was accompanied by an increased rate of drinking in the changed component, but drinking in the constant component did not decrease. These results are not consistent with the competition theory of contrast, but are consistent with the predictions based on the matching law. However, no current theory seems to account for all instances of behavioral contrast.Key words: behavioral contrast, behavioral competition, matching theory, interim responding, multiple schedules, bar pressing, rats Reynolds (1961) found that when reinforcement rate was reduced in one component of a multiple schedule, response rate in the other, constant component increased. This effect is known as positive behavioral contrast. Hinson and Staddon (1978) demonstrated that the magnitude of behavioral contrast increases when there is an alternative to operant responding. Rats' food-reinforced responding on a multiple schedule showed more contrast when a wheel-running response was available than when it was not. These data are the ma-

Two experiments examined the effects of session duration on responding during simple variable-interval schedules. In Experiment 1, rats were exposed to a series of simple variable-interval schedules differing in both session duration (10 min or 30 min) and scheduled reinforcement rate (7.5 s, 15 s, 30 s, and 480 s). The functions relating response rate to reinforcement rate were predominantly monotonic for the short (10-min) sessions but were predominantly bitonic for the long (30-min) sessions, when data from the entire session were considered. Examination of responding within sessions suggested that differences in the whole-session data were produced by a combination of prospective processes (i.e., processes based on events scheduled to occur later in the session) and retrospective processes (i.e., processes based on events that had already occurred in the session). In Experiment 2, rats were exposed to a modified discrimination procedure in which pellet flavor (standard or banana) predicted session duration (10 min or 30 min). All rats came to respond faster during the short (10-min) sessions than during the first 10 min of the long sessions. As in Experiment 1, the results seemed to reflect the simultaneous operation of both prospective and retrospective processes. The results shed light on the recent controversy over the form of the variable-interval response function by identifying one variable (session duration) and two types of processes (prospective and retrospective) that influence responding on these schedules.