To assess the impact of isoproterenol, duration of tilt, symptom development and hemodynamic changes on the outcome of tilt table tests, 100 patients with syncope underwent successive 80 degrees head-up tilt for 10 min during infusions of 0, 2 and 5 micrograms/min of isoproterenol. All 15 patients with another cause of syncope had a normal test result and 66 (78%) of the 85 patients with syncope of unknown origin had a test that resulted in syncope or presyncope. Isoproterenol was required to produce syncope or presyncope in greater than 90% of positive tests and 66% to 80% of positive tests required a dose of 5 micrograms/min of isoproterenol. Without isoproterenol, symptoms did not develop until after greater than or equal to 4 min of head-up tilt. With either 2 or 5 micrograms/min of isoproterenol, the half-time of symptom onset was 0.7 to 1.9 min and the rate of symptom development did not depend on the dose of isoproterenol. During syncope, the mean heart rate, systolic blood pressure and rate-pressure product each decreased significantly from 132 +/- 21 to 67 +/- 25 beats/min, 117 +/- 19 to 60 +/- 16 mm Hg and 15.3 +/- 2.9 to 4.2 +/- 2.2 x 10(3) mm Hg/min, respectively. During presyncope, mean trough rate-pressure product (5.5 +/- 2 x 10(3) mm Hg/min) was significantly higher (p = 0.027) than during syncope.(ABSTRACT TRUNCATED AT 250 WORDS)