We studied the effect of changing drive on resetting of respiratory rhythm in anesthetized cats and in a model (Van der Pol) of a limit-cycle oscillator. In cats, rhythm was perturbed by brief mesencephalic stimuli. Stimulus time in the cycle (old phases) and times of onset of rescheduled breaths (cophases) were measured. Previous study [Paydarfar and Eldridge, Am. J. Physiol. 252 (Regulatory Integrative Comp. Physiol. 21): R55-R62, 1987] showed distinct types of phase resetting that depended on strength of stimuli. In this study, stimulus strength was kept constant, but respiratory drive was changed by increasing PCO2, by stimulating carotid sinus nerve, or by cooling intermediate areas of ventral medulla. Type 0 (strong) resetting occurred when respiratory drive was low, type 1 (weak) resetting when drive was high, and a phase singularity when drive was intermediate. Phase-resetting patterns generated by the model showed the same behavior when a drive parameter was changed. The findings support the idea that continuous limit-cycle dynamics underlie generation of respiratory rhythm. Increased respiratory drive, by increasing size of the limit cycle, reduces functional effectiveness of the same perturbing stimulus in causing phase resetting.