Annihilation and one-to-one entrainment of modulated parasystolic rhythms in humans has not been previously discussed. In 9 nonmedicated patients, it was possible to measure the intrinsic, parasystolic ectopic cycle length given by the intervals between 2 consecutive parasystolic beats without any interposed nonparasystolic beat. The corresponding values varied between 960 and 2,350 ms (corresponding to rates between 62 and 26 beats/min). In addition, modulation could be determined, because nonparasystolic beats falling during the initial 59% of the cycle prolonged the parasystolic cycle length (by 12 to 37.5%), whereas those that fell later in the cycle shortened it (by 9 to 25%). Plotting this prolongation or shortening as a function of the temporal position of the nonparasystolic beats in the cycle yielded biphasic response curves, of which 7 were symmetric and 2 asymmetric. In 2 patients, episodes of concealed one-to-one entrainment were initiated by late nonparasystolic (sinus) beats and, later on, terminated by early ventricular extrasystoles. In 2 other patients (and in 2 separate occasions) nonparasystolic beats, falling in part of the cycle located in between those of maximal delay and acceleration, produced pacemaker annihilation (cessation of automatic activity for the remaining monitoring time). Parasystolic annihilation and concealed entrainment may be one of the causes that can explain the large, spontaneous, day-to-day variability in the incidence of ectopic ventricular beats reported in Holter recordings. Nevertheless, future prospective studies performing interventions that can change the sinus and ectopic rates are required to corroborate our finding.