A pacemaker has means for producing first and second signals one of which changes rapidly in response to changes in exercise level but may inaccurately represent the appropriate pacing rate. Such signal may be derived from a vibration sensor. The second signal accurately represents the required pacing rate but changes slowly in response to exercise level and may represent any of a number of different physiological variables, such as respiratory rate. The signals may be derived from the same or separate sensors. A control circuit provides a pacing rate appropriate to resting conditions if the first signal has a value less than a threshold regardless of the value of the second signal. The pacing rate is elevated to a predetermined level, such as 95 beats/min, if the first signal exceeds the threshold but the second signal has a value indicative of a pacing rate less than the predetermined level. The pacing rate is further elevated to substantially equal that indicated by the value of the second signal when the first signal exceeds the threshold and the second signal indicates a rate higher than the predetermined rate.