Previous studies show that mitochondria adapt to changes of arterial PO2. The present study investigates heart and brain mitochondrial respiratory rate (RR) in 23 guinea pigs at varying blood COHb levels. Blood gases and [COHb] were recorded in all animals. Brain and heart mitochondria were isolated. State 4 RR, state 3 RR with glutamate-malate substrate, expressed as nm O2/min/nm cytochrome a+a3, respiratory control ratio(RCR) (state 3/state 4), and Ca++ uptake were measured. All preparations were well coupled with RCR 10.7±1.9 for heart and 8.3±1.2 for brain mitochondria. State 3 RR increased from 225 at 0% COHb to 500 at 40% COHb in the heart (r=.8) and from 260 at 0% COHb to 628 at 80% COHb in the brain (r=.7). There was an inverse correlation of state 3 RR and arterial O2 content. Mitochondrial activity increased from 190-525 as arterial O2 content decreased from 19-9 ml/dl (r=.9) and from 340 to 580 as arterial O2 content decreased from 16-4 ml/dl (r=.6) in heart and brain respectively. Ca++ uptake in heart and brain increased with increasing [COHb]. Blood flow was not measured but presumably circulatory adjustment failed to compensate for the decreased arterial O2 content, thus impairing O2 delivery to the tissues. These data suggest that increased mitochondrial activity with increased [COHb] and decreased O2 content, occurring in the presence of normal PaO2, represent an adaptive response to tissue hypoxia comparable to the changes reported during acute and chronic hypoxemia.