Several authors have shown that CN participates in the acquisition of motor conditioned responses (MCR), probably as the integrating structure. Most of CN's efferent fibers in the cat end in the entopeduncular nucleus (EPN), therefore the command for the motor pattern could be exerted through EPN, representing part of the efferent link of the neuronal circuitry involved in MCR. Thirty-two cats were trained to avoid actively an electrical stimulus when a series of flashes appeared, and to press a lever to obtain 0.5 ml of milk. After the cats reached the learning criterion for both responses, electrolytic lesions of the entopeduncular nucleus or internal capsule (IC) were made bilaterally. When the cats recovered their normal motor behavior, the conditioned sessions were resumed once a day, for 45-50 days. Both learning responses disappeared (p less than 0.01) in those animals with the largest EPN lesions. In contrast, for small EPN lesions, learned responses were absent only during the first 3 or 4 sessions, and then the level of responses increased each day. However, it never reached that of sham lesioned cats. On the other hand, IC lesioned cats showed no statistical differences with respect to sham lesioned animals. These data support the participation of EPN in the motor circuitry responsible for MCR.