Analysis of clinical ventricular ectopy has provided deductive evidence for the existence of concealed ventricular rhythms. Studies in the experimental animal have demonstrated that when ventricular extrastimuli are applied near the ventricular effective refractory period of consecutive sinus beats, extrastimuli may not be conducted after sinus beats immediately following the post-estrasystolic pause. This sequence may be due to a prolongation of the effective refractory period of ventricular muscle produced by the increased cycle length resulting from the post-extrasystolic pause. The purpose of this study was to extend these observations to the human ventricle in an attempt to produce concealed ventricular rhythms by programmed ventricular extrastimuli. Twelve patients with normal PR intervals and QRS complexes with a mean age of 49 years were studied with ventricular extra-stimuli at programmed intervals following normally conducted sinus impulses. Premature ventricular impulses were introduced after every sinus beat at intervals 10 to 120 msec outside the effective refractory period. In eleven patients without ventricular interpolation, stimuli applied within 40 msec of the effective refractory period produced from one to three concealed (non-conducted) ventricular stimuli following the post-extrasystolic pause; stimuli applied greater than 40 msec outside the effective refractory period at identical current levels produced manifested bigeminy in all cases. In one patient with interpolated premature ventricular beats without a compensatory pause, programmed ventricular extrastimuli produced manifest bigeminy at all coupling intervals. This study demonstrates the experimental production of concealed and manifest ventricular rhythms in man and confirms the importance of the relationship between the ventricular effective refractory period, the timing of the premature ventricular impulse, and the occurence of a compensatory pause.