We have investigated the possibility that structural modifications of the sulfonium analogue of dopamine (4) would produce the same pattern of biological activity as structural modifications of dopamine. A series of methyl- tetralinyl -, and naphthalenylsulfonium analogues 5-7 were prepared and tested for their ability to inhibit the potassium-evoked release of [3H]acetylcholine from striatal slices. All compounds were tested under normal conditions and after depletion of dopamine stores with reserpine and alpha-methyl-p-tyrosine. The amine and sulfonium analogues 2-6 all showed direct agonist activity. The sulfonium analogue 7 produced, predominantly, indirect activity. In contrast to the amine analogues, chemical modifications of the sulfonium compounds produced little change in their dopamine agonist activity.