A detailed analysis was made of the changes in canine myocardium, with time of occlusion, in several important metabolites such as creatine phosphate and adenosine triphosphate (luminometry), inorganic phosphate (spectrophotometry), and most of the purines and nicotinamide adenine dinucleotide (high performance liquid chromatography). Within 1 min there was a significant reduction in creatine phosphate and a significant increase in inorganic phosphate, adenosine diphosphate, and adenosine monophosphate. A decrease in adenosine triphosphate became apparent after 4 min, concomitant with a progressive rise in the nucleosides, which reached almost 50% of the total purines after 64 min of occlusion. The formation of hypoxanthine was detectable in 50% only of all animals, suggesting a lack of active nucleoside phosphorylase in the others. Nicotinamide adenine dinucleotide, although decreasing slightly, was by far the most constant of all variables measured during at least 30 min of ischaemia. Therefore, this component is suggested to be a useful internal standard, thus minimising analytical and biological variations. Mioflazine, a potent nucleoside transport inhibitor (I50 3 X 10(-8) mol X litre-1), when given orally at 2.5 mg X kg-1, did not affect any of the changes with the exception of the nucleosides, where the drug completely inverts the adenosine to inosine ratio. The contribution of adenosine to the total nucleosides changed from 20% in the controls to 80% with treatment during at least 16 min of occlusion, there being no overlap between the groups. It is concluded therefore that adenosine is not deaminated in the cell where it is produced. It is not yet clear how this notable effect of mioflazine could be linked to its remarkable protective effect against ischaemia.