The understanding of the effects of cannabinoids in human subjects has been obscured by a lack of knowledge about how the various active principles from marijuana act at the cellular level in the brain. For this reason the present study was undertaken to determine the effects of cannabinoids on the enzymes associated with the synaptic membranes. Electron micrographic analysis was performed to determine the purity of synaptic membrane preparations from rat brain, and subsequently such preparations were subjected to additions of ethanol, Tween-80, 80% glycerol, and eitherΔ9-tetrahydrocannabinol, 11-hydroxy-Δ9-tetrahydrocannabinol, or cannabinol. Both sodium and potassium activated ATPase (Na+,K+-ATPase), and Mg2+—ATPase were measured as theµM orthophosphate (Pi) released per minute perµg membrane protein and these specific activities of the enzymes expressed as absolute values and as the percentage depression brought about by the cannabinoids. The ATPase specific activities are taken from the rate curve over a 30-min incubation time. Additionally, synaptic membrane acetylcholineesterase specific activity was measured by continuous rate enzyme assay. While as low as 10−7Mδ9-tetrahydrocannabinol showed appreciable decrements in both the membrane-bound ATPases, the other cannabinoids did not show such a great depression in enzyme activity. The specific activity of acetylcholinesterase, which is weakly bound to the membrane, showed only slight or no changes in activity with the various cannabinoids. It was additionally shown that the cannabinoids,Δ9-tetrahydrocannabinol in particular, bound to the synaptic membranes almost irreversibly in thein vitrosystem, and that the vehicle for dissolving the cannabinoids, while used as background control values when calculating the percentage decrements in enzyme specific activity, did vary the effects on the ATPase enzymes in particular. These data are discussed in relation to psychotomimetic activity of the cannabinoids.