The aim of this investigation was to study whether and to what extent 7-chloro-1,3-dihydro-1-methyl-5-phenyl-2H-1,4-benzodiazepin-2-one (diazepam, Valium) passes into the breast milk during repeated oral administration to nursing mothers. It was shown that diazepam and its main and also active metabolite desmethyldiazepam passed into the milk. Their concentrations, however, remained small as compared to plasma values. Desmethyldiazepam levels in the milk were consistently higher than those of diazepam. These results are explained by the relative binding of both substances to plasma proteins. The maximum quantities of diazepam + desmethyldiazepam that can possibly be absorbed by a newborn with the milk of a mother receiving 10 mg Valium daily, are considered to be too small to cause any untoward effects in the baby. If, however, high doses of Valium must be administered repeatedly, breast-feeding should be discontinued for safety reasons.