In rats injected in the cerebral ventricle with 250 micrograms of 6-hydroxydopamine a significant blockade of the improvement of swimming performance due to training occurs. The central injection of 6-OHDA leads to a lengthening of the total latency time (LT) of active conditioned reaction in shuttle box and of the LT of netative escape responses during learning, but not of the positive avoidance responses. 6-OHDA injected after the learing schedule (12 days) has no effect on the LT during relearning and extinction. Therefore a partial destruction of central monoaminergic, and especially of dopaminergic, neurons impairs particularly the acquisition but not the retention of the motor program.