[Development of the response of rabbit fetal tissues to thyroxine]

Development of rabbit fetal tissues sensitivity to thyroxin was studied on the basis of changes in alpha-glycerophosphate oxidase activity in the liver mitochondria of pregnant rabbits and their fetuses to which thyroxin was administered at different stages of pregnancy. T4 administration up to the 23rd day of gestation caused an increase in the enzyme activity in the maternal liver mitochondria only, whereas in the fetus it was at the control level. From the 24th day of pregnancy an increase of the alpha-glycerophosphate oxidase activity was seen in the fetal liver mitochondria. This can be explained by the appearance of rabbit fetal tissues sensitivity to maternal thyroxin at this period of gestation, increasing by the end of pregnancy.