The thiamine diphosphate content of liver mitochondria of female adult, pregnant, lactating and fetal and newborn rats has been measured. The liver mitochondria of fetal rats contain more thiamine diphosphate (0.40 ± 0.07 nmoles/mg protein than those of pregnant rats (0.26 ± 0.05 nmoles/mg protein). After birth, the thiamine diphosphate content of liver mitochondria increases by a factor of 2, gradually decreasing to the adult level (0.44 ± 0.06 nmoles/mg protein). Intraperitoneal injection of rats with thiamine dihydrochloride increased the thiamine diphosphate content of the liver mitochondria of the fetuses more than that of the mother anima