1. Four dogs were trained to immerse their snout voluntarily for durations up to 30 sec. Indwelling instrumentation was implanted to measure blood flow velocity in the circumflex branch of the left coronary artery, to sample blood from the left atrium and coronary sinus for the determination of P(O2), P(CO2), pH, oxygen saturation and haemoglobin concentration, and to pace the heart. An index of myocardial oxygen consumption was calculated by multiplying the mean flow velocity by the arteriovenous difference in oxygen content.2. Mean coronary flow velocity decreased significantly during simulated diving by 26 +/- 27% (+/- S.D.). The range of decrease in seventeen out of twenty experiments was from -5 to -81%. Heart rate decreased by 48 +/- 7% and this bradycardia was abolished by I.V. atropine.3. Coronary sinus oxygen saturation increased significantly with snout immersion (three dogs) and arteriovenous difference decreased from 67 +/- 10 to 47 +/- 5%. The index of myocardial oxygen consumption decreased by 42 +/- 19%. This decrease was attenuated slightly by beta-blockade but was abolished by cardiac pacing in three out of four experiments.4. The present study indicates that the heart consumes oxygen at a considerably reduced rate during simulated diving and therefore plays a direct role in the overall conservation of oxygen. This response appears to result primarily from a negative chronotropic effect induced by increased vagal tone.