In order to determine the effects of protracted nursing in American women, blood was collected hourly for 24 h and nursing periods recorded in 20 mothers, 10 amenorrhoeic, 3 3/4 to 17 1/4 months postpartum (PP), and 10 menstruating, 5 1/4 to 46 months PP. These data were compared to the daytime nursing behaviour and 1000–1100 h PRL of women among Kung hunter-gatherers of Botswana, a non-contraceptive using population with a birth space interval of < 3 years. Intense nursing behaviour maintained amenorrhoea and hyperprolactinaemia for 1 to nearly 2 years PP in both American and Kung mothers. Among Americans, 80 min of nursing per day, in conjunction with a minimum of six nursing episodes, was highly predictive of remaining amenorrhoeic up to 18 months PP. Amenorrhoea was always accompanied by hyperprolactinaemia, but delay in the onset of menses was related more to nursing behaviour than to a particular 24 h PRL level. The 1000–1100 h sample is equivalent to and about half of the 24 h mean in high and low intensity nursers, respectively. The Kung women were similar to the high intensity nursing American women in 1000–1100 h PRL, percent amenorrhoeic, and the number of minutes of daytime nursing.