An implantable medical system, comprising: one or more pulse generators configured to selectively produce pacing pulses; one or more electrodes (104, 106, 711, 768, 821, 822, 823, 824, 825, 827, 282, 830) configured to deliver pacing pulses produced by at least one of the one or more pulse generators to a heart of a patient within which the implantable medical system is implanted; a temperature sensor configured to produce a blood temperature signal indicative of a core body temperature of the patient within which the implantable medical system is implanted; and a controller (120, 186, 220) configured to: produce a relative temperature signal based on the blood temperature signal; produce a moving baseline temperature signal based on the relative temperature signal, wherein the moving baseline temperature signal follows the relative temperature signal during an initial drop in the blood temperature signal that occurs during an onset of activity, and the moving baseline temperature signal decays towards zero during an upswing in the blood temperature signal that follows the initial drop in the blood temperature signal; produce a proportional response signal based on the relative temperature signal and the moving baseline temperature signal; produce a sensor indicated rate response signal based on the proportional response signal and a base rate; and control delivery of the pacing pulses to thereby control a pacing rate based on the sensor indicated rate response signal. one or more pulse generators configured to selectively produce pacing pulses; one or more electrodes (104, 106, 711, 768, 821, 822, 823, 824, 825, 827, 282, 830) configured to deliver pacing pulses produced by at least one of the one or more pulse generators to a heart of a patient within which the implantable medical system is implanted; a temperature sensor configured to produce a blood temperature signal indicative of a core body temperature of the patient within which the implantable medical system is implanted; and a controller (120, 186, 220) configured to: produce a relative temperature signal based on the blood temperature signal; produce a moving baseline temperature signal based on the relative temperature signal, wherein the moving baseline temperature signal follows the relative temperature signal during an initial drop in the blood temperature signal that occurs during an onset of activity, and the moving baseline temperature signal decays towards zero during an upswing in the blood temperature signal that follows the initial drop in the blood temperature signal; produce a proportional response signal based on the relative temperature signal and the moving baseline temperature signal; produce a sensor indicated rate response signal based on the proportional response signal and a base rate; and control delivery of the pacing pulses to thereby control a pacing rate based on the sensor indicated rate response signal. produce a relative temperature signal based on the blood temperature signal; produce a moving baseline temperature signal based on the relative temperature signal, wherein the moving baseline temperature signal follows the relative temperature signal during an initial drop in the blood temperature signal that occurs during an onset of activity, and the moving baseline temperature signal decays towards zero during an upswing in the blood temperature signal that follows the initial drop in the blood temperature signal; produce a proportional response signal based on the relative temperature signal and the moving baseline temperature signal; produce a sensor indicated rate response signal based on the proportional response signal and a base rate; and control delivery of the pacing pulses to thereby control a pacing rate based on the sensor indicated rate response signal.