This investigation was designed to test whether the composition of free fatty acids varied in the milk of a group of marginally nourished women, throughout the day in response to their habitual pattern of eating. Ten fully lactating women, aged 19 - 29 years, weighing 53.3 ± 7.7 kg, witha mean height of 145.5 ± 4.5 cm volunteered for the study. They ate two standardízed meáis a day at 8 and 16 h. Milk samples were pumped out every 4 h, for a period of 24 h, starting at 8:00 A.M. Milk fat was extracted with hexane and the fatty acid content was determined in a gas chromatograph. No changes were noted in the mean diurnal pattern of individual fatty acids throughout the day. Saturated medium chain (MCFA), and long-chain fatty acids (C16:0, C18:0) were signifícantly lower and unsaturated long-chain fatty acids (C16:l, C18:l, C18:2) (UFA) were higher than that of reference well nourished women. Analyzed by individual cases, the changes in the sum of UFA were followed by mirror changes in MCFA. Diurnal peaks of total fat were associated with the peak of UFA, whereas troughs of total fat were associated with tróughs of UFA but peaks of MCFA. It is suggested that the low total milk fat reported in these women might be the combined result of a limited intake of dietary fat and a limited ability of the mammary tissue to synthesize de novo fatty acid.