Nitroglycerin ointment (GTNO) is widely used in the treatment of angina pectoris. In order to use this agent effectively, its duration of action must be known. This study was designed to determine how long GTNO acts in the ambulatory subject. A portable blood pressure measuring device was used for this purpose. 10 normal subjects were studied on 3 consecutive days: day 1, control (c); day 2, placebo (P); day 3, GTNO. The dose of GTNO used was the amount required to produce a > 10-mm Hg fall in systolic blood pressure (BP). There was no significant difference in BP, heart rate (HR) or BP X HR product between C and P. On day 3 (GTNO), a fall in BP (p < 0.001) occurred when compared with C and P, and this change remained significantly different from C and P over a period of 7 h. Since these data suggest that the action of GTNO is maintained for at least 7 h, we conclude that its administration at this interval should be effective in the relief of angina pectoris.