The hydroxyproline content of the amnion and the umbilical vessels obtained from cases in which the fetal membranes were instrumentally or prematurely ruptured was assayed. Hydroxyproline concentration (in μg/mg) in the latter group was 23.76, 16.79 and 17.86 in the amnion, artery and vein, whereas in the former the corresponding values were 42.20, 26.61 and 32.48. These results show that, in addition to a decrease in the amnion, hydroxyproline is lowered also in the umbilical vessels, suggesting that the reduction in the collagen in the fetal membranes is probably a particular manifestation of a general metabolic deficiency.