The concentration of taurine in rat milk is very high for the first few days after birth and then falls rapidly. [35S]Taurine injected intraperitoneally into lactating dams after birth was transferred via the milk to the pups, and accumulated in the brains of the pups to a greater extext than in the livers of the pups. Maximal accumulation of [35S]taurine so transferred to the brain of the pups was reached by 5 days after birth, and remained constant for at least 10 days beyond this poiht. The specific radioactivity in the brain of the pups also reached a maximal value at 5 days after birth and thereafter declined because of the expanding pool of unlabeled taurine in brain. At 5 days after birth, each pup has received approximately 4 mumol taurine from the mother via the milk, and a minimum of 7% of the total taurine in brain at this time originated from the milk. Speculation Even in the rat, a species which can synthesize taurine very easily from cysteine and methionine precursors, a significant amount of performed taurine is transferred to the developing animal via the milk. We suggest that the human infant, who cannot synthesize adequate taurine from cysteine and methionine precursors (9, 10, 40), may be dependent on its diet as a taurine source. Human milk contains a high concentration of taurine, whereas synthetic formulas contain virtually none. Taurine may be an essential nutrient for the rapidly growing human infant (and may be for the adult human also) and perhaps should be included as a supplement in synthetic formulas.