The study of vasoactive agents in man often requires accurate measurement of short-lasting changes in blood pressure. Using a noninvasive photoplethysmographic device (Finapres), the authors investigated in normotensive subjects whether rapid increases in blood pressure can be assessed precisely by monitoring finger blood pressure continuously. Six volunteers were studied on two consecutive days. On the first day, increasing doses of angiotensin I were injected intravenously with the aim to find a test dose which raised systolic blood pressure by 25 to 40 mm Hg. After oral administration of a placebo, the same test dose was injected repeatedly over the next 24 hours. On the second day, the subjects took either 6.25 (n = 3) or 25 mg (n = 3) captopril PO and the serial administration of the test dose of angiotensin I was continued for the next 4 hours. After placebo intake there was a good reproducibility of the blood pressure response to angiotensin I with a coefficient of variation of 15 +/- 4.5% (Mean +/- SD, n = 6). Captopril caused a dose-dependent inhibition of the pressor effect of angiotensin I. These data indicate that noninvasive blood pressure monitoring at the finger represents a useful tool to study short-lasting blood pressure changes produced by vasoactive agents in man.