In these experiments, we examined the prolactin (PRL) response during the acquisition of a conditioned avoidance response (CAR). Rats were tested daily in a two-way shuttle box. They were presented with a light stimulation followed by an electric footshock. During each trial period, the rats were given the opportunity to escape the footshock by moving to a safe side of the box. Movement to the appropriate location after the warning signal (light) begins, but before the onset of the footshock, constitutes a CAR. Blood samples were collected from an indwelling cannula and analyzed by radioimmunoassay. PRL levels increased during early acquisition testing, when the rats had not learned to avoid the shock. After one week of testing, acquisition performance increased considerably (70% CARs) while PRL levels remained unchanged. Thus, we were able to show that as rats learned to modify their behavior in response to a stressful situation, they could also modify their PRL response to the stressor.