Syncope in apparently normal patients has been attributed to an inhibitory reflex originating in cardiac sensory receptors. The reflex may be elicited by upright tilt with or without isoproterenol infusion. In this study, an upright 90 degree tilt protocol was evaluated in 20 young patients aged 7 to 22 years with syncope but with normal cardiac and neurologic evaluations. The electrocardiogram and blood pressure were noninvasively recorded at 1 min intervals while the patient was supine (5 to 10 min) and during tilt (15 min) in the baseline state. The protocol was repeated during isoproterenol infusion at increasing doses until symptoms of syncope or near syncope were provoked or the maximal isoproterenol dose was achieved (0.07 to 0.1 microgram/kg per min). Mean heart rate, mean blood pressure and RR interval variability, expressed as the standard deviation and the mean of the absolute difference between consecutive RR intervals, were assessed. Symptoms were elicited during tilt in 16 of the 20 patients (in 4 at baseline and in 12 with isoproterenol infusion); no symptoms were induced in 4 patients.(ABSTRACT TRUNCATED AT 250 WORDS)