The effects of marihuana and cannabinoids on natural and experimental models of epilepsy are reviewed. The psychoactive constituent of cannabis, Δ9-tetrahydrocannabinol, paradoxically exerts both a convulsant and anti-convulsant action. For example, it provokes myoclonus, psychomotor and grand mal seizures in epileptic beagles; however, at comparable doses it blocks maximal electroshock seizures in rats. In contrast, cannabidiol only exerts the anticonvulsant effects and lacks convulsant and psychotropic action. The anticonvulsant action of cannabinoids may result from decreased neural excitability and suppression of post-tetanic potentiation. The convulsant action of Δ9-tetrahydrocannabinol may result from a production of hypersynchronous neural discharge. Given these findings, epileptics should be discouraged from using marihuana since there is some risk of provoking seizures. However, because of anticonvulsant potency and lack of convulsant or psychotropic action, cannabidiol should receive clinical trials with epileptic humans as a test of its anticonvulsant effectiveness.