Basal metabolic rate (BMR), thyroid hormones and protein utilization were measured in six young men participating in a 75-d metabolic study of low zinc intakes. During metabolic period (MP) 1 (12 d) and MP 3 (9 d) the subjects received 16.5 mg Zn/d. During MP 2 (54 d) they received 5.5 mg Zn/d. Nitrogen intake averaged 12.6 g/d. Mean BMR significantly decreased from 1.00 +/- 0.06 to 0.91 +/- 0.05 kcal/(kg X h) (mean +/- SD) with the low zinc diet and tended to increase during period 3. Serum thyroid-stimulating hormone, thyroxine (T4) and free T4 tended to decrease during the low zinc period and to increase when the adequate zinc diet was fed; only the decrease in free T4 was significant at P less than 0.05, however. Among the measures of protein status, urinary urea excretion, serum prealbumin, albumin and retinol-binding protein levels all decreased significantly during the low zinc period. The results suggest that low zinc intakes may be associated with decreases in BMR. In addition, decreases in thyroid hormone levels and alterations in protein utilization may occur.