We compared the safety, efficacy, and side effects of induction of anesthesia with propofol (2.5 mg/kg), a new intravenous agent, and thiopental (4.0 mg/kg) in 62 patients in American Society of Anesthesiologists class I or II. There was no significant difference between induction times for the propofol (40.0 +/- 2.0 sec) and thiopental (44.0 +/- 4.0 sec) groups. Propofol administration produced a significant fall (P less than .05) in systolic blood pressure (SBP), from 134.1 +/- 2.6 mm Hg before injection to 128.3 +/- 2.4, 118.2 +/- 2.7, and 114.4 +/- 2.8 mm Hg one, two, and three minutes after injection, respectively. Diastolic blood pressure (DBP) fell significantly (P less than .05) during the three postinjection periods. Heart rate (HR) rose significantly (P less than .05), from 78.6 +/- 3.1 beats per minute before injection to 89.4 +/- 3.4 beats per minute one minute after injection. In patients given thiopental, SBP fell significantly (P less than .05), from 131.7 +/- 2.7 mm Hg before induction to 126.4 +/- 3.4 and 126.9 +/- 4.0 mm Hg two and three minutes after injection, respectively. The DBP did not change significantly in the thiopental group, but the HR rose significantly (P less than .05), from 73.3 +/- 2.8 beats per minute before injection to 83.9 +/- 3.0, 90.1 +/- 2.3, and 84.2 +/- 2.4 beats per minute one, two, and three minutes after injection, respectively. In 94% of patients given propofol, there were apneic periods of more than 60 seconds, compared to 50% in the thiopental group (P less than .05). There was a significant difference (P less than .05) between groups for the incidence of pain on injection; 31% of the patients receiving propofol had pain, compared to 3% of those receiving thiopental.