The physiological significance of taurine in milk in the growth of rat pups was investigated. Our results confirmed that taurine was at an exceptionally high concentration in rat milk during the lactational period, especially for the first few days after birth. Pups taking no milk from natural dams but from foster mothers at an advanced lactational period showed a slower growth rate. Intraperitoneal administration of taurine to the foster mothers in the first five days restored this growth retardation. On the other hand, intraperitoneal administration of β-alanine, a transport antagonist of taurine, to the natural dams through the lactational period induced a slower growth rate of pups. This β-alanine treatment to dams increased β-alanine concentration, but did not decrease taurine concentrations in milk, and serum taurine concentration in the pups receiving this milk was elevated. Direct administration of β-alanine to pups also increased the serum taurine concentrations dose-dependently. β-Alanine administration to pups significantly decreased [³H]taurine incorporation into all the organs examined, and in contrast, [³H]taurine concentrations in serum and urine were elevated. Thus, β-alanine inhibited taurine incorporation into cells and accelerated taurine excretion into either urine or milk. Serum IGF-I levels in pups receiving β-alanine either directly or via their mothers was significantly lower than those in control pups. Cumulatively, taurine ingestion from milk at an early lactational period seems critical for normal growth of rat neonates due to its role in maintaining normal serum IGF-I levels.