Pharmacokinetic studies of the concentration of the beta 1-blocker metoprolol have been performed in maternal plasma, amniotic fluid, breast milk and the plasma of the newborn. The concentration of metoprolol in maternal plasma exceeds that in the amniotic fluid initially but not later on. The quotient between the metoprolol concentration in the venous maternal blood and the mixed cord blood at the time of delivery is at about 1. In the maternal plasma the metoprolol concentration will decline in a rapid way after the latest dose probably because of an increased clearance. Blood levels of metoprolol during the first postnatal hours will increase almost fourfold and are generally followed by a decrease over the next 15 hours. Active metabolites of metoprolol (alfa-OH-metoprolol and O-demethyl-metoprolol) are found in the urine of the newborn. The breast milk concentration of metoprolol is 3 times higher than in the maternal milk but the sucking newborn will only show very low or unmeasurable plasma levels between consecutive breast feeding periods in the majority of cases.