Previous studies examining the relationship between response rate and reinforcement rate on variable-interval schedules (the variable-interval response function) have confounded elapsed session time with within-session changes in food density. The present experiments attempted to manipulate these factors independently and thus isolate their effects on responding. In Experiment 1, 7 rats pressed a bar for food on a series of four variable-interval schedules (7.5 s, 15 s, 30 s, and 480 s). Elapsed session time was held constant while food density was manipulated via a presession feeding. Changes in food density altered the form of the variable-interval response function, independently of elapsed session time. In Experiment 2, 8 rats responded on the same series of variable-interval schedules as in Experiment 1, but food density was held constant and elapsed session time was manipulated via the use of timeout periods. The results revealed no evidence for an effect of elapsed session time independent of food density. The present results extend a recent analysis of the variableinterval response function by Dougan, Kuh, and Vink (1993) by identifying food density as an important factor determining the form of the function. The present results also help clarify the controversy over the correct empirical form of the variable-interval response function by further defining the variables responsible for differences in the form of that function.Key words: variable-interval response function, satiation, food density, session time, within-session effects, simple schedules, bar press, ratsThe quantitative description of operant behavior has been an important focus of research for over 30 years. One of the earliest and most successful of these quantitative approaches was the attempt to describe behavior on concurrent variable-interval (VI) schedules in terms of the matching law (Baum, 1974;Herrnstein, 1961Herrnstein, , 1970 see Davison & McCarthy, 1988, for review).Herrnstein (1970, 1974)