There have been many experiments carried out in which a variety of spontaneously beating cardiac preparations have been subjected to periodic stimulation with a train of current pulses. There have also been many numerical studies on the response of limit-cycle oscillators to periodic forcing. Theoretically, there are basically three classes of rhythms that can be observed when a limit-cycle oscillator is driven with a periodic input: (1 ) periodic rhythms, in which there is a phase locking of the oscillator to its input, with the input stimuli falling at well-defined phases of the oscillation; (2) quasi-periodic rhythms, which are nonperiodic rhythms in which there is a gradual progressive shift in the phase of the oscillation a t which the stimulus falls; and (3) aperiodic rhythms, which are associated with chaotic dynamics. In several instances, the response of a cardiac preparation to premature stimulation has been used to predict its response to periodic stimulation.'-* The effect of injecting a premature stimulus into a spontaneously beating preparation is to phase-reset the rhythm of oscillation. Winfree has stated that there are essentially two qualitatively different kinds of phase resetting in biological oscillators: type 1, which is seen at low amplitudes of stimulation, and type 0, which is seen at high amplitudes of stimulation.' We have previously shown in experiments on embryonic chick ventricular cellslO~ and in numerical work on ionic models of Purkinje fiber12 and the sinoatrial nodei3 that, when type 1 resetting occurs, the new phase is a monotonically increasing function of the old phase, provided that the stimulus amplitude is sufficiently low. For higher stimulus amplitudes, the function becomes nonmonotonic, but the phase resetting remains type I . For a sufficiently high amplitude of stimulation, type1 phase resetting is no longer seen: it is replaced by type-0 phase r e ~ e t t i n g . ' ~ ' ~ We describe below the behaviors seen in a simple model, and in experiments on heart cell aggregates, when periodic stimulation a t such high amplitudes is applied. Both the modelingI5 and the experimental work have been previously described.