In this paper, we summarise the physiology of lactation and discuss the pathophysiology brought about by fasting, stress and anaesthetic drugs. Drug secretion into breast milk and subsequent absorption by the infant is considered. Maternal hydration must be well maintained with intravenous fluids, allowing an added 500 to 1000 ml for daily fluid loss in lactation. Maternal premedication, general anaesthesia and routine postoperative analgesics are also discussed as to the effects on the breast-fed infant. Drug side-effects may be avoided by timing breast feeding just before the next due dose. Sedatives with long half-lives should not be used. Endocrine and metabolic responses to anaesthesia and surgery are less with regional anaesthesia than with general, hence regional anaesthesia is preferred where it is a reasonable alternative technique.