In Exp. I, rats' lever presses were conditioned on multiple variable-interval variableinterval schedules. Changing one of the multiple schedule components to variable time reduced responding in that component. Further reductions in responding occurred in both components when the schedule was changed to multiple variable-time variabletime. After reinstating the multiple variable-interval variable-time schedule, lower response rates were maintained in the variable-time component during a series of stimulus reversals. In Exp. II, replacement of extinction components of multiple variable-interval extinction or multiple extinction extinction with variable-time schedules for single sessions (probe) resulted in response rate increments in those components. In the former schedule these increases were concomitant with response decreases during the variable-interval components. Response increases in the variable-time probes were related to conditioning history and, as a result, to response probability at the time of the probe.Positive reinforcers may be delivered dependent upon a specified response or independently of an animal's behavior. Zeiler (1968) designated schedules employing response independent reinforcers as time schedules (e.g., fixed time or variable time) to differentiate them from analogous interval schedules employing response dependent reinforcers. If behavior is conditioned on interval schedules, the usual finding is that response rates decrease when time schedules are substituted. For example, following conditioning on multiple fixed-interval fixed-interval (Appel and Hiss, 1962) or multiple variableinterval variable-interval (Weisman, 1970) schedules, pigeons were shifted to multiple fixed-interval fixed-time or multiple variableinterval variable-time schedules. Responding during the time components decreased across several sessions, while the rates of response in the interval components showed no system-

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AbstraetFour albino rats were trained to depress a lever on a DRL schedule. Reinforcement was delivered 10 seco after a correct response. A tone immediately followed a correct response. The Ss learned the DRL performance by reducing initially long IRTs rather than increasing initially short IRTs as is usually the case when reinforcement is not delayed. ProblemDelayed reinforcement has been shown to retard the rate of response in a free operant bar pressing situation (Perin, 1943;Dews, 1960; Azzi, et aI, 1964;Skinner 1938). Since DRL schedules require that responding be decreased there was reason to believe that delayed reinforcement might aid Ss in the acquisition of DRL performance. It also seemed likely that a stimulus change after a correct response would help to keep response rate from dropping off too sharply for the performance to be acquired (cf., Ferster, 1953). MethodThe Ss were four male albino rats, 100-110 days old at the beginning of the experimento After each session they were fed a sufficient amount of Purina lab chow to keep them at 80% of their ad lib feeding weight. Water was continuously available in the home cage.Two-hr sessions were scheduled five days a week. During the first session each S was run on a CRF schedule with a 2-sec. delay of reinforcement. A tone (Tone 1, Foringer Stimulus Control Panel, ~o. 1166-4) was on continuously during this delay period. Starting with the second session all Ss we,re placed on a DRL 20-seco schedule in which a correct response produced a 2-sec. tone. A 45 mg food pellet was delivered 10 sec. after the response (Le., 8 sec. after the termination of the tone). The tone was extended to the entire 10-sec. delay interval from session 28 to session 36. R ... suIts Figure 1 shows the modes and interquartile ranges (Q) of each S's performance in the manner described by Hodos (1963)0 Responses emitted during the first 2 sec. of the inter-response time (IRT) were not included in the tabulations since they seem to represent behavior which is not actually under schedule control. Several aspects of the performance are shown clearly in Fig. 1