Variations of plasma ionized calcium and pH were studied in 16 children anaesthetized with halothane and succinylcholine for minor surgery. During anaesthesia, a close negative correlation was noted between pH and ionized calcium. Intubation was followed by a fall of the ionized calcium concentration. A mean decrease of 0.04 mmol X L-1 was noted, without pH change. The acute elevation of carbon dioxide following extubation allowed the in vivo effects of pH on ionized calcium to be studied. For each pH unit decrease, a mean rise of 0.60 mmol X L-1 of ionized calcium was found. During the study, no significant variations of plasma total calcium concentration were observed. The variations in the hydrogen ion concentration and the administration of succinylcholine were accompanied by changes of the free calcium ion concentration, due probably to alterations in calcium binding to proteins, and perhaps to the formation of a calcium complex with anions. Our results emphasize the importance of knowledge of the pH for the interpretation of ionized calcium variations in the presence of acid-base disturbances.