The responses of plasma insulin and the C-peptide of proinsulin, glucose, lactate, free fatty acids (FFA), glycerol, D-beta-hydroxybutyrate and alanine to a beta2-adrenergic agonist (salbutamol) were determined in 5 patients during the last trimester of pregnancy before labor. Salbutamol was given as an infusion in the same dosage as is used to inhibit uterine contractions in cases of premature labor and in obstetric emergencies. The influsion of salbutamol was given for 45 min, accompanied by repeated sampling of arterial blood. All patients developed moderate tachycardia and exhibited metabolic effects following salbutamol infusion, implying marked increases in plasma levels of insulin, C-peptide, glucose and lactate. An increased rate of lipolysis was evident from the rises of FFA, glycerol and D-beta-hydroxybutyrate. Plasma levels of alanine declined, possibly due to stimulation of gluconeogenesis. It is unlikely that these acute maternal metabolic changes would have significant adverse effects on the fetus.