Abstract 1. Dimethyldopamine was eight times more potent than dopamine in activating the D 2 receptor that inhibits the potassium-evoked release of [ 3H]acetylcholine from striatal slices. 2. Cocaine and mazindol produced an eight-fold shift in the concentration-response curve for dopamine, but not for dimethyldopamine. 3. The IC 50 of dimethyldopamine for the inhibition of [ 3H]dopamine uptake was thirty times greater than that for dopamine. 4. Dopamine may be less potent than dimethyldopamine at the D 2 receptor because dopamine has a higher affinity for the dopamine uptake system, resulting in its rapid removal from the vicinity of the receptor.