Human milk zinc concentrations from women of low socioeconomic status in Ribeirao Preto, Southern Brazil, were studied. A group of 23 mothers, 17 multiparae and six primiparae aged from 17 to 39 years, were included in a 6-month longitudinal study. A total of 192 samples were collected starting from colostrum and then at 15-day intervals until the end of the sixth month of lactation. Initial zinc values (0.47 +/- 0.10 mg/dl) fell by about 40 per cent during the second week of lactation to 0.28 +/- 0.06 mg/dl, the decrease progressively increasing up to the 24th week (0.11 +/- 0.03 mg/dl). Zinc values were significantly higher in the colostrum than in any of the later collections (P less than 0.05). The values obtained at 15 days were significantly higher (P less than 0.05) than those obtained later, except for those obtained on the 30th day, which were higher than all of those, obtained later (P less than 0.05). The values obtained after 75 days of nursing, although steadily decreasing, were not statistically different. The data of the present study demonstrate a significant reduction in zinc levels during lactation, the amount of zinc provided by breast-milk being lower than those recommended by the recommended dietary allowance in the United States.