1. The effect of different doses of ouabain, an inhibitor of the sodium pump, or saline (9 g sodium chloride/1; the vehicle) on the metabolic rate of guinea-pigs weighing 500 g was measured by indirect calorimetry for 120 min.
2. Ouabain (0·02–0·07 nmol/g body-weight) decreased the oxygen consumption in a dose-related manner. When higher doses of ouabain (0·10 nmol/g body-weight or greater) were injected the animals were observed to shiver. Ouabain (0.07 nmol/g body-weight) reduced Na+, K+-ATPase (EC 3. 6. 1. 3) activity of liver, kidney and skeletal muscle by 18·0 (se 6·6), 21.5 (se 6·0) and 21·9 (se 6·8)% respectively. An Eadie-Hofstee plot of percentage decrease in O2 consumption v. percentage inhibition ÷ dose of ouabain showed that maximal inhibition of O2 consumption was 39·4 %.
3. It is concluded that Na pump activity contributed to about 40 % of the resting O2 consumption.