Milk volume and composition, nutrient intake and plasma prolactin levels of 22 well-nourished lactating women were measured during a 3-week period; an experimental group (E, n = 14) reduced energy intake by 19 to 53% (mean 32%) during week 2 while controls (C, n = 8) maintained normal intake. Mean maternal energy intakes (kcal/day) were: (E) 2316 (wk 1), 1591 (wk 2); (C) 2037, 2121. Mean infant milk intakes (g/day) during these weeks were: (E) 736, 718, 684; (C) 714, 671, 702. Milk intake was not reduced among infants of mothers whose energy intakes were at least 1500 kcal/day during week 2. In group E prolactin levels tended to increase during week 2. Milk composition did not change significantly. Lower energy intakes than those currently recommended appear compatible with successful lactation and gradual maternal weight loss, but longer-term studies are needed to determine a lower limit for safe intakes.