Direct arterial pressure, heart rate, and ECG have been recorded over a 24-hour period in nine individuals who were completely unrestricted throughout the study. Forty-nine separate cigarette smoking episodes were clearly indicated and analyzed. The results of our study confirm a significant increase in arterial pressure five minutes after smoking a cigarette. The systolic rise in pressure (mean 10.7 mm. Hg, P less than 0.001) was approximately twice that of the diastolic rise (5.3 mm. Hg, P less than 0.001) and was present under different conditions of everyday life with notable exception of lying in bed before sleep, We found no quantitative difference between normotensive and hypertensive subjects. There was no certain change in heart rate (mean increase +0.8 beats per minute, t equals 0.59, NS) in the group as a whole. Smoking also had a short-term action consisting of a brief fall in arterial pressure and heart rate occuring over eight to ten heart beats following immediately after the first inhalation of tobacco smoke, followed by a rebound rise in arterial pressure to a level greater than the presmoking level; this is probably a vagal effect. Cigarette smoking caused angina pectoris in one individual and the records showed ST-segment depression in the ECG before the subjective appreciation of pain.