Little is known about the placental transfer of indomethacin in the human pregnancy. Twenty-six pregnant patients (gestational age, 29.4 +/- 0.5 weeks) were given a 50 mg oral dose of indomethacin 6.08 +/- 0.07 hours before 42 cordocenteses undertaken for standard indications. Maternal serum, fetal serum, and amniotic fluid levels were measured at the time of each procedure. Maternal indomethacin levels were not significantly different from corresponding fetal levels (218 +/- 21 vs 219 +/- 13 ng/ml). The maternal/fetal serum ratio (0.97 +/- 0.07) was not found to vary with gestational age (R = -0.07, p = 0.66). Fetal serum levels were significantly higher than corresponding amniotic fluid levels (219 +/- 16 vs. 21 +/- 2 ng/ml; p less than 0.001). The fetal/amniotic fluid ratio (10.0 +/- 1.2) did not vary with gestational age (R = 0.33, p = 0.11). Indomethacin crosses the human placenta easily throughout gestation; only small amounts of the unchanged drug are found in the amniotic fluid.