Cannabinoids have been implicated in a variety of cognitive processes in humans, including attention, learning, memory, and time estimation. However, studies of the effects of cannabinoids on rodent behavior have focused on motor, learning, and memory tasks. To assess cannabinoid effects on time perception, this study examined whether systemically administered cannabinoid receptor agonists and a cannabinoid receptor antagonist influenced rats' performance of a time interval estimation task based on a fixed-interval schedule (a "peak procedure"). Both cannabinoid agonists WIN 55,212-2 and delta9-tetrahydrocannabinol shortened the modal response time, and cannabinoid antagonist SR 141716A lengthened the modal response time. Secondary measures of the shape of the response distribution were not influenced by any of the drugs, suggesting that the response distribution shifts were not artifacts of drug side effects. Therefore, these experiments argue for the involvement of endogenous cannabinoids in time estimation.