It may be difficult to evaluate sinus bradycardia in the elderly since sinus rates below 50 beats/min may occur normally with aging. Seven asymptomatic bradycardic subjects (heart rates 41-51 beats/min), ages 67-79 years, with no evidence of impaired cardiac performance and taking no drugs, were compared with four age-matched controls (heart rates 60-84 beats/min). Autonomic function was tested with the Valsalva maneuver, amyl nitrite inhalation, methoxamine infusion, head-up tilt, and atropine intravenously (2 mg).Cardiac index at rest was normal in bradycardic subjects (2.4 ± 0.1 SEM); increased cardiac stroke volume (SV) compensated for the slow heart rate. With supine bicycle exercise, increase in cardiac output per 100 ml increase in oxygen consumption was normal in bradycardics (885 ± 60 ml compared to 907 ± 171 ml in controls). This exercise response was achieved mainly by increased SV in bradycardics and by increased heart rate in controls. Significant increases in cardiac output occurred with both atropine and atrial pacing in bradycardics, but not in controls. Autonomic impairment was not found. Increased vagal tone in bradycardics was suggested by a lesser heart rate response to exercise and passive tilting.Hence, a heart rate below 50 beats/min in elderly people does not necessarily indicate depressed cardiac performance. Further evaluation of the significance of bradycardia in these subjects requires long-term observation.