High doses of proprandolol administered to rats during pregnancy and lactation cause inhibition of growth of the neonatal rats. Growth remains inhibited if drug treatment is continued after weaning. Cessation of drug administration after weaning leads to a nearly normal growth rate but a relatively long period elapses before normal weight is attained. The administration of large doses of propranolol to young growing rats also causes inhibition of growth. This inhibition is quickly reversed upon cessation of drug treatment.