The results from the previous work (Bukoski et al, 1983) and those presented here indicate that mitochondria from rat heart are capable of producing adenosine in concert with a sarcolemmal 5'-nucleotidase. In addition, and perhaps more importantly, these data also indicate that mitochondria can produce adenosine in the presence of inhibition of sarcolemmal 5'-nucleotidase. Given the current understanding of the regulation of mitochondrial respiration and thus oxygen consumption, mitochondria may be uniquely situated to sense increases in myocardial oxygen demand and respond to this with a feedback signal (adenosine) which can return the system to a state of balance.