Seven patients with threatened premature labor and six nonpregnant female control subjects were studied for 12 to 24 hours during ritodrine infusion in order to re-evaluate the effects of this beta-adrenergic tocolytic agent on blood anions, electrolytes, and acid-base balance. Dexamethasone was given intramuscularly in all patients and three control subjects at the beginning of ritodrine infusion. Lactate, pyruvate, betahydroxybutyrate, sodium, potassium, chloride, bicarbonate, pH, glucose, and insulin levels were measured in blood samples obtained at intervals of 2 to 6 hours. While pH remained in the normal range, increases in lactate and pyruvate levels and decreases in bicarbonate values resulted in a significant increase in anion gap 4 to 6 hours after the beginning of treatment. The blood potassium level fell simultaneously. These effects were transient, lasting less than 10 hours, despite continuation of ritodrine infusion and maintenance of tocolysis and tachycardia. Although larger increases in the blood glucose level were observed with combined therapy in control subjects, dexamethasone had no additional influence on potassium concentrations. Monitoring of patients treated by intravenous ritodrine should include routine measurements of blood glucose and electrolyte levels in order to detect its potentially harmful metabolic side effects for mother and fetus.