Of 25 primiparous women, investigated prospectively for six months post-partum, 19 persisted with breast feeding and six changed to artificial during the first six weeks. Sexual activity, mood, and feeding patterns were recorded in weekly diaries. Hormones were measured from weekly urine samples (oestrogen and pregnanediol) and fortnightly blood samples (prolactin, testosterone, androstenedione, and sex hormone binding globulin). In breast-feeding women, testosterone and androstenedione levels were significantly lower in those who reported severe reduction in sexual interest. Changes in sexuality or mood were not related to levels of prolactin or oestrogen, or to the return of follicular activity, which was delayed in persistent breast feeders. The relationships of mood, sexuality, and hormones are discussed.