The involvement of the cerebellum and the red nucleus in the classically conditioned nictitating membrane/eyeblink response in the rabbit is investigated using direct stimulation of the interpositus or the red nucleus as the unconditioned stimulus. Stimulation of either of these structures produced eye closure in naive animals, and this eye closure was paired with a tone in the standard Pavlovian conditioning paradigm. The results indicate that eyelid closure due to stimulation of either the red nucleus or the interpositus nucleus is not sufficient for the development of conditioned responses to the tone. Animals which had received interpositus stimulation as the unconditioned stimulus acquired the conditioned response to tone significantly faster following the substitution of air puff for stimulation than did those animals that had received red nucleus stimulation, or controls that did not receive any stimulation. However, animals that had been trained to tone-air puff could not retain the conditioned response after being switched to tone-interpositus stimulation. Lesions of the interpositus and the red nucleus through the stimulating electrodes were effective in impairing or abolishing conditioned responses. The results are interpreted to indicate that the red nucleus and interpositus are elements of the circuit that carries out the expression of the conditioned response. In addition the interpositus, but not the red nucleus, may be critical in the formation of the memory trace for the conditioned stimulus-unconditioned stimulus association, by virtue of the greatly accelerated learning that results from its stimulation.