The contribution of fetal tracheal fluid in the formation and bilirubinoid staining of amniotic fluid was evaluated in fetal rabbits. Fetal tracheal fluid samples were obtained from extrauterine rabbit fetuses maintained in a hyperbaric oxygen chamber. Samples of amniotic fluid were taken from the tail and head region of intrauterine fetuses following placement of a peri-uterine suture at the level of the fetal neck. Amniotic and allantoic fluids were also obtained at various stages of pregnancy and after tracheal obstruction. These fluids were subsequently analyzed for total volume, rates of formation, spectral absorption, bilirubin and chloride content. This study indicated that fetal tracheal fluid contains bilirubinoid pigments including indirect bilirubin and that tracheal fluid is a possible route of fetal pigment excretion into the amniotic fluid. Extrapolations of these conclusions are made to man in erythroblasto-sis fetalis and fetal distress.