Measurements of dietary zinc and copper absorption obtained after administration of a single dose of the extrinsic stable isotopic tags 70Zn and 65Cu were compared to measurements made with standard chemical balance methods in 41 appropriate for gestational age premature infants [body wt 1267 +/- 258 g, gestational age 29.8 +/- 1.9 wk (mean +/- SD), 4 to 83 postnatal d of age]. Fifty studies were performed; 33 with premature formula, five with term formula, seven with preterm human milk (PTHM), and five with fortified-PTHM. The percentages of net zinc and 70Zn absorption were found to be significantly greater from PTHM (66.4 +/- 15.2, 68.6 +/- 9.8) than from premature formula (14.0 +/- 29.9, 31.6 +/- 22.4), and term formula (23.6 +/- 18.5, 17.6 +/- 5.6). The percentages of net copper and 65Cu absorption were also found to be significantly greater from PTHM (61.5 +/- 14.0, 69.8 +/- 14.0) than from premature formula (16.6 +/- 20.6, 39.6 +/- 21.6) and term formula (20.6 +/- 24.1, 26.5 +/- 6.9). The percentages of net zinc and 70Zn absorption (35.9 +/- 29.1, 48.4 +/- 9.6) and net copper and 65Cu absorption (38.7 +/- 10.2 and 57.4 +/- 13.1) from fortified PTHM were similar to values from PTHM. Absorption of zinc and copper determined with extrinsic stable isotopic tag and standard nutrient balance methods were significantly correlated. Estimates of endogenous fecal losses of zinc and copper were substantial with each diet, but lower with PTHM. Stepwise, multiple linear regression analysis accounted for, at most, 58% of the variability in the measures of zinc and copper availability. We conclude that extrinsic 70Zn and 65Cu tags can be used to study absorption of dietary zinc and copper by very low birth wt infants.