There is concern that the breast-fed infant whose mother is receiving intra-muscular progestogens for contraception will be exposed to significant quantities of the steroid. Norethisterone levels in maternal serum and milk were studied throughout an injection interval after intramuscular administration of 200mg norethisterone oenanthate. Milk samples were taken at the beginning and at the end of the feed. Norethisterone concentrations in milk were very much lower, but declined more slowly, than serum concentrations. Post-suckling concentrations were higher than pre-suckling concentrations. Experiments in adults receiving an oral dose of norethisterone in cow's milk comparable to that ingested by an infant in a day resulted in low serum levels. It is concluded that only very low concentrations of norethisterone would be present in the infant's circulation.