Platelet functions are becoming the useful tool for delineating the etiology of pregnancy induced hypertension. Electronmicroscopic studies, efflux and content of 5-HT in platelets and platelet aggregation responses towards various aggregating agents have been measured in 25 normotensive pregnant subjects and 31 PIH subjects. Marked decrease changes have been noted in aggregation parameters with transformation from discoid to "spiny sphere" of platelets with long pseudopods along with prolonged time for spontaneous aggregation by platelets in PIH. Serotonin release from platelets was decreased and reserpine-induced release-reaction showed enhanced kinetics in PIH. Platelet serotonin content was raised and was inversely related to platelet count with severity of syndrome. Hence, a check and balance for platelet activation and aggregation in PIH might be crucial in the development of PIH.