Ahn To investigate whether estrogen deficiency may impact the platelet aggregation function in fe-male SD rats and the underlying mechanisms relative to nitric oxide (NO).Methods Female SD rats were randomly divided into three groups: sham, ovariectomy, and ovariectomy supplemented with estrogen group. Each of them received sham operation, ovariectomy or ovariectomy followed by estrogen supplementation respectively. Estrogen supplementation started 3 days after the ovariectomy and continued for the following 4 weeks, then the animals were sacrificed. The serum was obtained to perform the estrogen assay. The platelet-rich plasma was collected to measure its platelct aggregation abil-ity, and to detect plasma cGMP (Guanosine 3, 5-cyclic monophosphate), the NO bioactive index, via the method of ELISA. Results The serum estrogen level in ovariectomized female SD rats was much lower than that in shamed ani-mals, estrogen supplementation effectively restored the ovariectomized animals' serum estrogen. The platelets aggregation in ovariectomized rats was remarkably higher than that in shamed animals, estrogen supplementation could lower the plate-lets aggregation ability, though not to the sham level. The plasma cGMP level in the ovariectomized rats was significantly lower than that of shamed animals, estrogen supplementation may remarkably enhance the plasma cGMP, to a level in keeping with the sham group. Conclusion Lack of estrogen may boost the platelets aggregation in female SD rats, the mechanism might be relative to the down-regulation of NO/cGMP by estrogen depletion.