The dramatic endocrinologic changes that occur after childbirth and the coincident affective disturbances that range from "maternity blues" to major depression have stimulated much theorizing and some study. The role of breast-feeding and weaning has received remarkably little attention both in the more biologically oriented studies and in epidemiologic work. This paper reviews endocrinologic data which support the thesis that postpartum psychiatric disorders have a hormonal basis and discusses the possible psychiatric effects of breast-feeding and weaning. The cases of four patients who developed major depressions in close temporal association with weaning are presented and discussed.