Total parenteral nutrition solutions supplemented with cysteine-HCl (S-TPN, 0.8 mmol/kg/day) were infused into beagle pups from day 10 of life to day 20 (n = 6). Another group of pups received unsupplemented TPN solutions (US-TPN) (n = 6). Fluid, protein, and energy intake from nonprotein sources were similar in both groups. Data from these two groups were compared and similar measurements in normally suckled pups were also compared with the two TPN groups (n = 6).There were significant differences in the rate of weight gain between the pups that received TPN and the pups that were suckled (P > 0.01). Weight gain, hepatic DNA and protein concentrations, and cerebral DNA and protein concentrations in the pups that received TPN supplemented with cysteine were not different from similar measurements in pups that received unsupplemented TPN. Plasma total cyst(e)ine (Cyst(e)ine refers to the mixture in any proportion of the sulfhydryl (cysteine) and the disulfide (cystine) forms of this compound) concentrations in the cysteine-supplemented pups (7.9 ± 1.2 μ/DL, x ± SD) were significantly greater than in the unsupplemented pups (4.9 ± 1.8 μmol/DL). Hepatic glutathione concentrations in the supplemented pups (583 ± 85 μmol/100 g liver) were also significantly greater than in the unsupplemented pups (392 ± 113 μmol/100 g liver). These data suggest that the supplementation of TPN solutions with cysteine, even in an animal enzymatically capable of cysteine synthesis, has significant effects on glutathione synthesis.