The ability of amniotic membrane, obtained with 3 h of delivery of 28 infants, to interconvert 3H-cortisol and 14-C-cortisone has been examined. The conversion of cortisone to cortisol increases as a function of gestational age, being first seen at 29.5 weeks and reaching values of 50--55% at term (n = 28, r = 0.615, P less than 0.01). Furthermore, a net gain of the active hormone (cortisol) resulting from the bidirectional interconversion of cortisol and cortisone ("C-11 activation index") appears at 31 weeks and reaches 40--50% by term (n = 28, r = 0.674, P less than .001). We speculate that this phenomenon could be an important source of active glucocorticoid for the fetus.