[14C]inosine in a range of concentrations of 20 microM to 1 mM was administered to the isolated perfused rat heart for 30 min. The incorporation of the nucleoside into myocardial adenine nucleotides increased for extracellular concentrations of the precursor up to 50 microM, reaching a plateau at 60 nmol . g-1 X 30 min-1 with concentrations ranging between 50 and 200 microM. The supply of 500 microM and 1 mM of inosine induced a further increase in cardiac adenine nucleotide synthesis to about 200 nmol . g-1 X 30 min-1. When supplied during low flow ischaemia (0.5 mL . min-1, 30 min.), 1 mM of inosine protected the heart against ATP degradation, while 100 microM of inosine was inefficacious. In the presence of 1 mM of inosine on reperfusion the adenine nucleotide content of the heart was similar to that observed in the absence of the nucleoside. The incorporation of [14C]inosine into adenine nucleotides was, in this last condition, below the value measured before ischaemia. Inosine administration was effective in protecting the heart against ischaemic breakdown of glycogen and favoured postischaemic restoration of glycogen stores.