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Acute marijuana administration may alter response–reinforcer relationships via a change in reinforcer efficacy, but may also impair coordination and motor function. One approach to evaluating drug effects on both motor function and reinforcer efficacy involves fitting the matching law equation to data obtained under multiple variable interval (VI) schedules. The present report describes an experiment that examined the effects of acute marijuana on response properties using this approach. Six human subjects responded under a multiple VI schedule for monetary reinforcers after smoking placebo and two active doses of marijuana. The low marijuana dose produced unsystematic changes in responding. As measured by the matching law equation parameters (k and rB), at the high dose five subjects showed a decrease-motor-related properties of response rate and four subjects' responding indicated a decrease in reinforcer efficacy. These data raise the possibility that, at high doses, marijuana administration alters both motor function and reinforcer efficacy.