1. A bioassay for cannabis, called the ring test, has been developed in which the percentage of the total time spent on a horizontal wire ring during which a mouse remains completely immobile is recorded.2. The effect of cannabis on mobility is a dose-related, graded response.3. Threshold doses of cannabis extract are 12.5 mg/kg when injected intravenously, and 100 mg/kg when injected intraperitoneally or subcutaneously.4. The method provides a measure of the ;cataleptic' effect of cannabis. Chlorpromazine in doses of 1 mg/kg upwards also produces the effect but barbitone does not.5. It is concluded that Delta(1)-tetrahydrocannabinol (Delta(1)-THC) is largely responsible for the effect of cannabis extract on mobility; the potency ratio of Delta(1)-THC to cannabis extract is between 10 and 20. Delta(1)-Tetrahydrocannabidivarol (Delta(1)-THD) also affects mobility but is less active than Delta(1)-THC. Cannabidiol has no effect when injected intraperitoneally in doses up to 100 mg/kg.