1. The effect of bicarbonate administration on the intracellular pH of rat skeletal muscle was examined by using 31P n.m.r. 2. Bicarbonate administered intraperitoneally caused a significant intracellular acidosis in rat skeletal muscle in vivo. When the bicarbonate was administered intravenously there was no such change in the pH of the skeletal muscle. 3. Bicarbonate administration by either route resulted in an elevated mixed venous partial pressure of carbon dioxide and an elevated arterial pH, but no significant change in the arterial partial pressure of carbon dioxide. The increase in arterial bicarbonate concentration after intraperitoneal injection of bicarbonate was delayed when compared with that after intravenous injection. 4. The administration of hypertonic solutions intravenously caused a transient 40-50% fall in blood pressure, which had resolved within 1 min. 5. The data suggest that the effect of bicarbonate administration on intracellular pH in vivo is related not only to carbon dioxide loading of the cell but also to the rate of change in the extracellular bicarbonate concentration.