The effective refractory period of the right ventricle (ERP-V) was measured in 27 patients during atrial or ventricular pacing using the ventricular extra stimulus method. Pacing was conducted with impulses of 1.5-2 times diastolic threshold. The ERP-V was directly related to the basic cycle length (BCL) although the ERP-V was always greater for atrial pacing than for ventricular pacing at a given BCL. The ratio ERP-V/BCL was greater at shorter cycle lengths indicating that a larger fraction of the cycle was refractory at faster heart rates. The ratio ERP-V/QT interval did not change over a range of BCLs, but the ratio was larger for atrial pacing (.77 +/- .05 SD) than for ventricular pacing (.60 +/- .05). Atropine (1 mg i.v.) was given to six patients. The drug did not affect the ERP-V in six of eleven determinations, prolonged ERP-V twice and shortened it slightly three times. Measurements were reproducible over an hour but varied at a given BCL when measured on separate days. Asymptomatic repetitive beating occurred in seven of 27 patients when the premature stimulus was within 20 msec of the ERP-V. The effective refractory period of the right ventricle in man can be determined reproducibly and with safety. Changes induced by various perturbations parallel results from in vitro single cell and myocardial studies.