The study examines the effects of nursing practices on the duration of lactation in middle-class American women. Thirty-two nursing mothers were followed for 2 years post-partum, data being collected at eight home visits by interview and by nursing records kept by the mothers. Those women who nursed frequently (>8/day) during exclusive breastfeeding remained amenorrhoeic longer than infrequent nursers, introduced supplements later and did not resume menses as promptly thereafter. They continued an hour or more of night nursing during supplemented nursing. Duration of exclusive nursing and night nursing after supplementation were the major influences on duration of amenorrhoea. Mothers' age, weight-for-height, and nursing frequency before supplementation showed no significant effect but night nursing after supplementation was a major factor in post-supplementation duration of amenorrhoea. Those women who both supplemented later and maintained an hour of night nursing after supplementing remained amenorrhoeic for 6–10 months longer than those who supplemented early and/or reduced subsequent night nursing.