Transfer of 3H-cortisone and 3H-cortisol between the amniotic, allantoic, and fetal blood compartments in the rabbit at day 25 of gestation.

We have examined the rate of disappearance and the form of the radiolabel following the injection of either [1,2-3H]-cortisone (3H-E) or [1,2,6,7,n-3H]-cortisol (3H-F) into the amniotic sac of rabbits at day 25 of gestation. The half-life of 3H-E was approximately 35 min and was very much shorter than that of 3H-F. Concurrent with the disappearance of 3H-E was an increase in the amount of tritium in both the allantoic and the fetal blood compartments. Whereas following injection of 3H-E there was a rapid conversion of E to F that was reflected in all three compartments, following injection of 3H-F the vast majority of the tritium label remained as F. We conclude that amniotic E may act as a ready source of fetal F.