In 2K,1C-RHR (two-kidney, one-clip hypertensive rats) serotonergic mechanisms in blood platelets were studied. The endogenous serotonin (5-HT) concentration in whole blood and in platelets remained unchanged in relation to the sham operated rats. Also the uptake of labelled 5-HT in rats with renal hypertension was not altered. However platelets aggregability was increased in 2K,1C-RHR. Acute administration of captopril (10 mg/kg and 100 mg/kg po) diminished blood pressure but did not change either the concentration of 5-HT in whole blood and in platelets of hypertensive rats or the uptake of this amine. Platelets aggregation and the amplifying effect of 5-HT in hypertensive rats were also unchanged after acute captopril administration. Similar results were observed after its administration in a dose of 30 mg/kg for one week. Our results indicate that captopril did not affect the platelets serotonergic mechanisms in 2K,1C-RHR.