The sympathoadrenal and renin-angiotensin systems play an important role in the cardiovascular responses induced by head-up tilt. In young people, hypotension and postural changes induced by tilt produce significant increases in plasma renin activity. The response of plasma renin activity to prolonged tilt and subsequent vasovagal hypotension has not been assessed in the elderly. Seventeen elderly subjects (10 with carotid sinus syndrome and seven age-matched controls) were studied during 45 minutes of recumbency and 120 minutes of head-up tilt to 40 degrees. Intra-arterial systolic blood pressure and heart rate were monitored continuously and plasma renin activity was measured at frequent intervals. Ten subjects developed vasovagal syncope during tilt (at 32 +/- 18 min). Systolic blood pressure and heart rate fell by 70 +/- 20 mmHg and 20 +/- 7 beats . min-1 (P less than 0.001 and P less than 0.01, respectively). Plasma renin activity did not change in syncopal subjects and remained unchanged up to 60 min after syncope. In seven subjects who completed 120 min of tilt without symptoms, heart rate rose during tilt but plasma renin activity remained unchanged. Plasma renin activity is not influenced by marked hypotension or prolonged head-up tilt in the elderly.