Nitrogen(N) concentrations of human milk in various fractions, such as total, protein, nonprotein, whey protein and casein were determined at 2-3 days, 1, 2, 4, 6 and 12 weeks of postpartum. significant decreases in total N, nonprotein N, protein N, whey protein N and casein N were found with time postpartum. Total nitrogen decreased from 401mg/dl at 2-3 days to 211mg/dl at 12 week. Whey protein nitrogen was found to contribute to the total nitrogen decrease with time. The percentage of nonprotein nitrogen was 13% in colostrum and 17-18% in mature milk. The proportions of whey protein and casein nitrogen were 55:45 at 2-3 days and 34:66 at 12 week postpartum. These determinations will provide the basic information on the variability of nitrogen components as lactation proceed.