Compared to operated and nonoperated controls, rats with small bilateral lesions in the anteroventral caudate nucleus or the rostral substantia nigra were significantly impaired in the acquisition of one-way active avoidance, passive avoidance requiring the inhibition of the previously acquired oneway response, and shuttle-box avoidance. The animals with nigral lesions took significantly more trials to criterion than the animals with caudate lesions on one-way avoidance. Results were considered in terms of the intimate anatomical and neurochemical relationships between these structures, and a circuit of structures involved in avoidance learning, differing from one previously described, was suggested.