The contact phase of blood coagulation in a group of patients suffering from essential hypertension was studied before and after captopril administration. The baseline levels of factor XII, factor XI and plasminogen were significantly higher than in normals and correlated with baseline diastolic blood pressure levels. On the contrary, plasma prekallikrein was not significantly different from normal. These results suggest the presence of a hypercoagulable state in essential hypertension. After captopril administration, factor XII, factor XI and prekallikrein rapidly decreased, perhaps as a consequence of the drug's effect on the vascular endothelial surface. There was no correlation between the changes of active and inactive renin and the changes of prekallikrein and plasminogen levels. Our data do not support the view that factor XII-plasma kallikrein or plasmin dependent pathways are involved in the activation of inactive renin in vivo. Captopril, by provoking rapid pressure changes, appears to be able to affect the clotting system.