Estrogen (125 micrograms/rat) was administered, subcutaneously, to male rats six days before measurements were made. Estrogen treatment increased the number of dopamine receptor sites labeled by [3H]spiroperidol in vitro and the intensity of stereotypy induced by dopamine agonists in vivo. These two measures suggest that estrogen administration significantly influences the number of striatal DA receptors and a response to in vivo striatal DA receptor stimulation.