Reinforcement schedules restrict an organism's access to 1 activity (the contingent response, or reinforcer) by requiring it to engage in a 2nd activity (the instrumental response) for access to the 1st one. Behavior is also constrained by limitations of time, so that an increase in an activity entails a decrease in some others. If an organism's repertoire consists of N independent, mutually exclusive and exhaustive activities, these 2 constraints can be represented as surfaces in an N-dimensional space whose axes are the levels of the N activities. The distribution of activities under free conditions is represented by a point (the free-behavior point) in such a space. Functional relations between the equilibrium levels of the instrumental and contingent responses under different schedules of reinforcement can be generated by the homeostatic assumption that organisms act to minimize the distance between the point representing their distribution of activities under schedule conditions and the free-behavior point. This simplified approach predicts the form of the functional relations obtained on ratio, interval, and several other schedules, as well as the differences between them. (51 ref) (PsycINFO Database Record (c) 2006 APA, all rights reserved). © 1979 American Psychological Association.