The level of taurine and the activities of the two enzymes involved in its synthesis, cysteine dioxygenase and cysteinesulfinate decarboxylase, were measured in tissues of rat dams during pregnancy and lactation and in their fetuses or pups. The most marked changes observed in the dams included an increased hepatic taurine concentration in late pregnancy (20 days), a decreased plasma taurine concentration during late pregnancy and throughout lactation, and a decrease in tissue cysteine dioxygenase activity in late lactation (20 days). These observations suggest that the dam's taurine pools may be an important source of taurine for secretion in the milk and that pregnant dams may prepare for the onset of lactation by accumulating additional taurine in the liver. There was little if any correlation between the activities of either of these key enzymes in taurine synthesis and tissue taurine levels. The taurine concentrations in liver, brain, heart and plasma of the young decreased between 1 day before birth and 20 days of age. Cysteine dioxygenase and cysteinesulfinate decarboxylase specific activities increased in the brains of pups between birth and 20 days of age, and cysteinesulfinate decarboxylase activity increased in the livers of pups between birth and 20 days of age. Cysteinesulfinate decarboxylase activity in 80-day-old male rats was more than six times that in female littermates, but no effects of sex on cysteine dioxygenase activity or tissue taurine concentrations were observed.