Concentrations of prostaglandin E (PGE), prostaglandin F (PGF) and 13,14-dihydro-15-keto-prostaglandin F (PGFM) have been measured in fetal tracheal and amniotic fluid from chronically catherized sheep during late pregnancy. Amniotic fluid contained significantly greater concentrations of these prostaglandins than tracheal fluid (p<0.01); there was no correlation between the level of prostaglandins found in each fluid. In tracheal fluid concentrations of PGE and PGFM exceeded those of PGF (P<0.01) whereas no significant differences were found in amniotic fluid. The levels of prostaglandins in these fluids were similar in ewes bearing hypophysectomized fetuses.