1. Milk and plasma metronidazole and hydroxymetronidazole concentrations were measured in 12 breast‐feeding patients following multiple doses of metronidazole (400 mg three times daily). All patients received metronidazole in combination with other broad spectrum antibiotics. 2. Plasma concentrations of both parent drug and metabolite were measured in seven suckling infants. Thirty‐five infants were monitored for adverse reactions to maternal metronidazole therapy and two further groups of suckling infants, those whose mothers received either ampicillin alone or no drug therapy, were recruited as controls. 3. The mean milk to plasma ratio (M/P) was 0.9 for metronidazole and 0.76 for hydroxymetronidazole while the mean milk metronidazole concentrations (around Cmax) were 15.5 micrograms ml‐1. The mean milk hydroxymetronidazole concentration was 5.7 micrograms ml‐ 1. 4. Infant plasma metronidazole concentrations ranged from 1.27 micrograms ml‐1 to 2.41 micrograms ml‐1, and the corresponding hydroxymetronidazole concentrations from 1.1 to 2.4 micrograms ml‐1. 5. There were no significant increases in adverse effects in infants which could be attributable to maternal metronidazole therapy. 6. Metronidazole was excreted in milk at concentrations which caused no serious reactions in the infants studied. The drug may therefore be administered at doses of 400 mg three times daily to mothers wishing to breast‐feed their infants.