Previously, several studies in our laboratories have linked the ability to initiate movement rapidly (reactive capacity) in several rat strains to markers of nigrostriatal dopamine function. In the present investigation we wished to determine the extent to which fast-reacting (FR) and slow-reacting (SR) rats responded differentially to the effects of dopamine agonist (apomorphine) administration. Fisher 344 rats were operantly conditioned on a shock-avoidance, reactive capacity task which requires extremely fast, ballistic, lever release responses. In FR, but not in SR rats, significant dose-dependent decreases in the per cent of successful avoidance trials were observed in response to apomorphine doses. Moreover, apomorphine brought responding in the FR animals to levels below that of the SR rats. Performance of the SR rats was unaffected by drug treatments. A number of interpretations are consistent with these preliminary data, including the idea that basal differences in speed of reaction and differential responses to a dopamine agonist, antiparkinsonian agent may both be associated with a similar mechanism: differential activation of compartmentalized striatal efferent systems.