The effects of ventricular pacing on the arrangement of ventricular parasystolic beats were studied in 14 patients. By analyzing the effects of various pacing rates and modalities, it was found that both rate and pattern of manifest parasystolic beats were intimately related to the rate and coupling interval of the paced rhythm. Our findings indicate that fixed coupling of ectopic beats is not incompatible with parasystole; modulated parasystole may manifest as fixed rate classic parasystole; a parasystolic pacemaker can be entrained by a wide range of driving rates both above and below the intrinsic rate of the parasystole; and with different driving rates, these rate-dependent patterns can be observed in the same patient. Our observations suggest that spontaneous or drug-induced changes in the heart rate can lead to major alterations in the frequency and patterns of ventricular parasystole.