Mothers among !Kung hunter-gatherers nurse briefly and frequently, with brief intervals between nursing bouts (mean +/- standard error, 13.19 +/- 1.28 minutes). The low levels of 17 beta-estradiol and progesterone in the serum of the mother are correlated with infant's age and with interbout interval, but not with total nursing time. Maternal gonadal function is apparently suppressed by a timing-dependent, prolactin-mediated effect of breast stimulation. Interbout interval may be a key variable in lactation infertility. If so, it solves the puzzle of !Kung birth spacing.