SUMMARY1. Adrenaline 1 μg markedly elevated the activity of adenylate cyclase in ventricular tissue of rat hearts.2. Δ1‐THC 25, 50 and 100 μg significantly reduced the adenylate cyclase activity with the maximum effect observed with the dose or 25 μg. Doses below or above this range did not produce any significant effect on the enzyme activity.3. Neither Δ6‐THC 25 and 100 μg nor PVP 400 μg had any significant action on adenylate cyclase.4. The Δ1‐THC‐induced lowering of the cyclic AMP concentration in ventricular tissue of rat hearts can be explained, at least partly, by its ability to reduce the activity of adenylate cyclase in these tissues. It is suggested that this enzyme inhibition underlies the cardiac depressant a