Morphine raised the levels of intravenously administered ampicillin in the plasma of mice. Despite higher ampicillin levels in plasma after administration of morphine, levels of this antibiotic in bile and urine were not elevated. After ligation of the common bile duct, ampicillin levels in plasma were elevated. Morphine caused a further rise in drug levels in plasma of duct-ligated mice. Ampicillin levels in plasma were higher in mice made anephric by prolonged ligation of their external urethras. In such animals, morphine also caused ampicillin levels in plasma to be even higher. These experiments suggest that morphine impairs both renal and hepatobiliary elimination of ampicillin. These effects of morphine were completely reversed by naloxone. In contrast to effects on intravenously administered ampicillin, morphine markedly reduced drug levels in plasma when ampicillin was given by gastric intubation. This resulted from delayed absorption because of retardation of gastric emptying by morphine.