When E manipulates the delay before reinforcement in free operant situations, he also manipulates the interval between the last response leading to the n - 1 reinforcement and the first response leading to the nth reinforcement. The present experiment studied effects of delaying reinforcement when the interval between these responses was not varied hv r! -, -. Three white rats of the Sprague-Dawley strain, maintained at 80% of free-feeding weight, were trained in a Lehigh Valley cage (Model 143-26) enclosed in a sound actenuating chamber. Relay and programming equipment were located in an adjacent room. Upon completion of a fixed-ratio of 45 responses (FR 45) the response lever retracted from the cage for 30 sec. A 45-mg. food pellet was delivered either 0, 5, or 10 sec. after lever retraction. Each S was given 15 daily sessions of 50 reinforcements each at each delay interval starting with the 0-sec. interval and increasing progressively in 5-sec. increments. Performance was reevaluated at the 0-sec. condition after all other conditions had been presented. Cumulative post-reinforcement pauses (time between entrance of the lever into the cage and S's first response) and responding time (time elapsing from the first response until the lever retracted) were recorded for the 50 trials of each session, and only data from the last five days at each condition were used in the analysis. With this rechnique reinforcement delay was varied within a constant interval (30 sec.) between S's last response before a given reinforcement and until he could resume responding after thac reinforcement. TABLE 1