The nerve terminals in the striata of rat brain were labeled in vitro with [3H]dopamine via the uptake mechanism for catecholamines. Subsequently, the striata were incubated with cocaine, nomifensine, or mazindol, inhibitors of catecholamine uptake. The tissues were rinsed in fresh medium and then stimulated with 20 mM potassium to induce release of [3H]dopamine. Under these conditions, each drug decreased the potassium-stimulated release of radioactivity by 40-50% compared to control tissues which had not been exposed to the drugs.