The sensitivity of rat platelets to the aggregating agent adenosine-diphosphate (ADP) was studied. Platelets form male and female rats demonstrated comparable sensitivity. Neither ovariectomy nor castration altered platelet sensitivity to ADP. However platelets form mock-operated female, but not mock-operated male rats were less sensitive. Administration of physiological doses of 17 beta-oestradiol to ovariectomised rats reduced platelet sensitivity. Administration of 17 beta-oestradiol to mock operated female rats increased platelet sensitivity to ADP. The results suggest that physiological amounts of oestrogen induce a loss of platelet sensitivity following surgery or stress, whereas higher concentrations of oestrogens increase platelet sensitivity.