We investigated the time course of cardiac mitochondrial O2 consumption following steps in heart rate in 16 isolated rabbit hearts perfused with Tyrode solution. The time course was characterized by the mean response time, i.e., the first statistical moment (mean time) of the impulse response function. Like the mean transit time for an indicator, it provides an important characteristic of the response time course. The venous O2 content transients during steps in heart rate were measured and corrected for O2 diffusion and vascular transport using a mathematical model with experimental information derived from O2 washout following steps in arterial O2 concentration or perfusion flow. We deduce from these washout experiments that the effective O2 solubility in heart tissue is 86 +/- 13% (mean +/- SE) of solubility in water. The measured venous mean response time following a step in heart rate at 37 degrees C was 17.6 +/- 1.1 s. The mean response time of cardiac mitochondrial O2 consumption to changes in heart rate after correction for O2 transport was 7.7 +/- 0.7 s.