1. Zinc metabolism has been studied in a group of undernourished, slum-dwelling, lactating women in Manaus, Brazil, by means of modified metabolic balance techniques and the stable isotope 67Zn.
2. The subjects were found to be consuming a diet which provided an average of 34% of the recommended dietary allowance for lactating women, but six of the seven appeared to achieve Zn balance. In five of the subjects use of 67Zn in a stable isotope dilution manner demonstrated that they were absorbing a high proportion of the dietary Zn (proportional absorption ranged from 0·59 to 0·84), suggesting an adaptation to the chronically low intake.
3. Two subjects had marginally low plasma Zn concentrations, although hair, urine and milk Zn contents were all within accepted normal values.
4. Preliminary findings on the rate of plasma Zn turnover and the size of the exchangeable body Zn pool obtained using 67Zn suggest that these subjects may have a reduction in both.