A controller for variably controlling the pacing rate of a cardiac pacer responsive to temperature which includes a logic and control unit, a parameter communication unit, an analog to digital converter and a temperature sensor. The temperature sensor in the right ventricle or atrium communicates a value related to blood temperature through the analog to digital converter to the logic and control unit. The logic and control unit operates under control of the rate algorithm to calculate a pacing rate value related to variations in the blood temperature. The pacing rate value is calculated as the sum of a reference rate, a natural rate response term, and a dynamic rate response term which contributes rate only in response to physical activity. A step rate response is also added to the calculated pacing rate when predetermined criteria related to the blood temperature and calculated pacing rate indicate the onset of physical activity. Under the control of the rate algorithm, the controller also maintains the level and rate of change of the calculated pacing rate within predetermined limits, and prevents the pacing rate from remaining at the maximum rate limit after physical activity has ceased.