The electrophysiologic effects of 45 degrees head-up tilt were studied in 19 patients with atrioventricular accessory pathways. Upright posture enhanced both anterograde and retrograde accessory pathway conduction when compared to the supine position: the anterograde block cycle length decreased from 374 +/- 52 ms (mean +/- standard error) (supine) to 303 +/- 33 ms (tilt) (p less than 0.05); anterograde effective refractory period decreased from 286 +/- 17 to 249 +/- 10 ms (p less than 0.05); retrograde block cycle length shortened from 331 +/- 36 to 291 +/- 35 ms (p less than 0.05); retrograde effective refractory period decreased from 312 +/- 26 ms to 274 +/- 15 ms (p less than 0.05). During induced atrial fibrillation the mean RR interval and the shortest RR interval between preexcited beats decreased approximately 10% with head-up tilt. During orthodromic reciprocating tachycardia, tachycardia cycle length shortened 15%. Tachycardia rate during electrophysiologic study in the head-up position more closely approximated the rate of clinical tachycardia than did the rate in the supine position. Head-up tilt significantly enhances anterograde and retrograde accessory pathway conduction, increases the rate of arrhythmias using an accessory pathway and may be clinically useful in the assessment of patients with an accessory pathway.