Unexpected cases of icterus gravidarum in infancy have in many instances been traced to drug absorption from the maternal circulation. After consulting with paediatricians, it was considered advisable to investigate under hospital supervision the effect, if any, of metronidazole (Flagyl) on the foetus and later on the breast-fed infant. In Edinburgh a and baby survey on 96 patients completed in December, 1960 (Scott Gray, 1961), suggested that there was no placental barrier to the drug. The serum concentrations in mother and child were approximately equal at the time of delivery. The drug was well absorbed during labour, and in four cases where gastric lavage had been previously carried out the serum concentrations were higher than in the remaining cases investigated. Serum bilirubin estimations performed on mothers and babies showed no appreciable increase. This paper presents the results of investigations into the secretion of metronidazole in breast milk and the absorption by the breast-fed infant.