The relative contributions have been determined of taurine derived from the mother in utero, via milk during nursing, and from endogenous biosynthesis to the total taurine content of the rat pup between birth and weaning. At birth, 32% of the taurine in the pup has been biosynthesized, and this proportion rises to 83% by day 20 of life. At birth, 67% has been derived from the mother in utero, and by day 20 this has fallen to 4% of the total. This maternal taurine is lost with a half-life of 16 days. There is wide variation in the turnover from different tissues, the pancreas having a half-life of 7 days, and the brain 50 days. However, the amount of maternal taurine in the brain actually increases by 38% over the first 8 days of life. By day 20, 13% of the taurine content of the pup has been obtained from the milk. Taurine turnover in the suckling pup differs from turnover after weaning in that whole-body turnover from the suckling rat is not slower than exchange between organs. In other words, tissues are not in kinetic equilibrium. After animals are weaned, regardless of the taurine content of the diet, taurine is interchanged between organs faster than it is excreted from the animal.