delta9-Tetrahydrocannabinol (THC) augments the locomotor activity produced by methamphetamine (0.5 mg/kg) in aggregated mice. THC-induced augmentation was dose related and lasted for a two-hour period. Maximal effective dosage of THC was 15 mg/kg with higer dosages of 30 and 60 mg/kg producing a decrease from maximum in locomotor activity. THC, 15 mg/kg, also increases locomotor activity among aggregated animals treated with saline. However, the increase was much less than the methamphetamine augmentation. In similar studies using isolated mice THC produced only a dose-related decrease in locomtor activity among both methamphetamine-treated and saline-treated animals. THC, 60 mg/kg, had no effect on methamphetamine-induced lethality in aggregated mice. However, at 15 mg/kg, THC significantly enhanced the lethality of methamphetamine. THC did not alter methamphetamine lethality in isolated mice. Distribution studies using 14C-methamphetamine indicated that neither THC nor isolation of animals affected tissue concentration or disappearance of 14C material. Previously reported synergistic interaction between amphetamine and THC is related to aggregation of the animals rather than drug treatment. Since THC at low doses can stimulate motor activity in saline-treated animals, amphetamine may act only to amplify the behavioral activity produced by low doses of THC.