Abstract: In a method and apparatus for operating an implantable cardiac pacemaker, the stimulation rate is modulated, while a patient in whom the pacemaker is implanted is experiencing a number of different levels of physical stress, and a number of signal sequences corresponding to a non-mechanical physiological characteristic are obtained, one sequence being obtained for each physical stress level. These signal sequences form a reference field, which is stored in the pacemaker. A current signal sequence of the non-mechanical physiological characteristic is subsequently obtained, and this signal sequence is compared to all of the signal sequences in the stored reference field, and one of the reference field signal sequences having the highest correlation with the current signal sequence is selected. The stimulation rate is then set at a rate equal to the basic stimulation rate for the modulation which produced the highest correlating sequence in the reference field.