The concentration of LH and FSH was measured in the fetal blood and cerebrospinal fluid (CSF) of 4 human fetuses between 21 and 24 weeks old. A comparison was made with 6 post menopausal females. In one fetus the serum and CSF levels of both hormones were determined after administration of 200 micrograms LRH and 400 micrograms TRH. The mean serum fetal levels of LH and FSH of 76.6 mIU/ml (range, 29.5-152.0) and 30.5 mIU/ml (range 3.1-60.3), respectively, were not significantly different from those of postmenopausal females, whereas the fetal CSF levels of these hormones were significantly higher (p less than 0.02) than those of post menopausal females. The serum to CSF ratios for the fetal LH and FSH were 4.6 +/- 1.1 and 2.4 +/- 0.61 respectively, significantly different (p less than 0.02 and p less than 0.001 respectively) from those of post menopausal females (29.1 +/- 6.6 for LH and 50.9 +/- 10.9 for FSH). In one fetus LRH did not provoke any change in circulating FSH and LH level 20 min. after administration, while a slight increase of these hormone in the CSF was observed. These data suggest that the permeability of blood-CSF barrier to gonadotropins in the fetus seems to be higher than in the adult who has no blood-CSF barrier (B--CSF--B) alterations.