A noninvasive isotopic method for the determination of milk intake in breast-fed infants was evaluated against the conventional method of test-weighing. In experiment 1, 48-h breast milk intake was estimated concurrently by the test-weighing procedure and by the deuterium dilution technique. In experiment 2, the isotopic method, modified to minimize fluctuations in water flux, was evaluated against a 24-h test-weighing. In experiment 1, the mean 48-h milk intake estimated by the isotopic method (1616 +/- 353 ml) was significantly higher than that measured by the test-weighing procedure (1449 +/- 234 ml) (p less than 0.01). In experiment 2, 24-h milk intake as determined by test-weighing and deuterium dilution averaged 878.0 +/- 188.1 and 690.7 +/- 141.4 ml/day, respectively, and differed significantly (p less than 0.001). The ability of the deuterium dilution method to predict values obtained by the test-weighing procedure was unsatisfactory for individual estimations. These experiments indicate that the deuterium dilution technique is unacceptable for the determination of breast-milk intake in individuals, but may be satisfactory for a population estimation.