Thymidine kinase activities, virtually all soluble in rat lung, liver, and small intestine, decreased abruptly late in gestation or immediately after birth. An injection of thyroxine delayed the fetal but not the neonatal changes in liver activity. An injection of cortisol decreased hepatic and pulmonary thymidine kinase activities in both fetal and neonatal rats but had little effect on the intestinal enzyme. Premature extrauterinization led to an earlier occurrence of the quantitative changes in thymidine kinase activity usually seen at term. Birth-associated changes included a rapid transitory increase in the hepatic enzyme and the virtual loss of intestinal thymidine kinase activity. In human tissues, the soluble thymidine kinase in liver remained high between the 11th and 22nd wk of gestation whereas the particulte enzyme, the predominant form in adult liver, rose in the second half of gestation and reached adult levels at birth. In human lung, the soluble enzyme started to decrease by the 16th gestational wk, whereas the particulate thymidine kinase reached the higher adult levels late in gestation. Thymidine kinase in adult human tissues was predominantly particulate.