A heavy-water dilution technique has been used to measure mean daily breast-milk output in a group of nursing mothers from an urban community in Santiago, Chile. Infant milk intake was found to correlate significantly with weight (r = 0.646, P less than 0.005) and with weight-for-age (r = 0.640, P less than 0.005), but a much stronger and highly significant correlation was found with infant birth-weight (r = 0.802, P less than 0.001). Milk output was also found to depend on the mother's nutritional status at the beginning of pregnancy and was significantly higher in overweight mothers. An even more marked difference was obtained if mothers were divided into two subgroups, above and below normal, according to their weight-for-height values at the first antenatal check-up (x+ = 1148.1 ml/d, x- = 814.2 ml/d, P less than 0.02). Relationships between milk output and duration of lactation and parity were also apparent from the study but the latter was probably mediated through maternal nutritional status.