Recent studies have shown that the law of supply and demand describes behavior on simple Variable Interval (VI) schedules. When the quantity of reinforcement supplied is large, animals will pay less for the reinforcer than when quantity supplied is small. These studies, however, feature organisms responding alone in operant chambers, without the social competition which economists argue drive the law of supply and demand. The present series of experiments examine the effects of social context on the economic behavior of rats on VI schedules. Rats responded on a pseudo-randomly assigned sequence of VI schedules differing in reinforcement rate. During half of the sessions, a second rat was placed in the chamber behind a Plexiglas barrier. As predicted by economic theory, there was an inverse relationship between the quantity of reinforcement supplied and the obtained behavioral cost of reinforcement. In addition, the presence of a competitor rat altered the relationship between supply and cost