Mice were implanted subcutaneously with a pellet containing 0.5 mg estradiol placebo. On the day following implantation platelet aggregation was produced in arterioles on the brain surface (pial arterioles) by injuring their endothelium in vivo with a noxious light/dye stimulus. The time between the onset of the noxious stimulus and the appearance of platelet aggregates was significantly shortened (p<.01) in the estradiol treated mice. In contrast to this enhancement of aggregation, when platelets were tested ex vivo in platelet rich plasma (PRP), aggregation to sodium arachidonate or to ADP was not altered in estradiol treated mice. Thus the enhanced aggregation observed in injured pial arterioles of estradiol treated mice may not reflect direct effects of estradiol on the platelet itself. Rather enhanced aggregation may reflect an effect of estradiol on endothelium or adjacent tissue. This effect was produced within 24 hours of achieving pregnancy levels of estradiol. The rapidity of the effect is of interest in light of earlier studies showing a similar in vivo action following 7-14 days of high estradiol levels. The rapid effect suggests that an action on megakaryocytes is an unlikely explanation for the results.