Microinjections of 6-hydroxydopamine (6-OHDA) into the dopaminergic nigrostriatal system (NS) of the rat impaired acquisition of a two-way avoidance response. This effect was independent of nutritional deficiencies since it was observed even when a special postoperative treatment ensured a comparable state of nutrition in control and experimental rats. It was likewise independent of locomotor disturbances. A third question remains open: Doses of 6-OHDA that impaired acquisition of an active avoidance response produced lesions that were, to a great extent, nonspecific. Therefore, behavioral effects of these doses cannot be entirely ascribed to selective destruction of the NS.