1. The effect of oestrogen-containing compounds on platelet factor 3, platelet life span, renal platelet localization and the fibrinolytic activity of forearm venous blood has been studied in eleven women with the renal vascular abnormalities of the loin pain and haematuria syndrome. The results were compared with those obtained in twenty-nine healthy female volunteer subjects. 2. Platelet factor 3 (PF3) availability was found to be increased in patients with the loin pain and haematuria syndrome during normal menstrual cycles. Oestrogens produced an increase in PF3 availability in both patients and control subjects. 3. Platelet life span was shortened and the index of renal platelet localization increased when patients with the loin pain and haematuria syndrome received oestrogens. No change in platelet kinetics was found in the healthy control subjects receiving oestrogens. 4. The fibrinolytic activity of forearm venous blood, both at rest and after venous occlusion, was increased in the healthy control subjects in response to oestrogens. No such response was seen in the patients with the loin pain and haematuria syndrome. 5. It is suggested that the change in platelet function may be causally related to the intra-renal vascular abnormalities. 6. The possible role of oestrogens in causing these disturbances or unmasking pre-existing abnormalities of platelet function or fibrinolysis is discussed.