Four quadrant quantitative amniotic fluid volume (length, width, and depth) was performed in six fetuses prior to and during indomethacin therapy for preterm labor in patients with normal amniotic fluid volume prior to therapy. The dose of indomethacin was 25 mg orally every 6 hours. One patient had to have the dose reduced to 25 mg every 12 hours due to constriction of the ductus arteriosus. Fetal urine output was determined prior to and during indomethacin therapy. The mean pretherapy amniotic fluid volume of 341.5 +/- 43.2 mm declined to 97 +/- 9.3 mm at the time of discontinuation of indomethacin for oligohydramnios. This occurred after 15 days of 25 mg indomethacin orally every 6 hours in four patients and after 28 days at 25 mg indomethacin every 12 hours in the remaining patient. Fetal urine output prior to and following indomethacin was 4.93 +/- 14 ml and 1.73 +/- 0.6 ml/hr, respectively. Prolonged indomethacin therapy results in decreased fetal urine output with resultant oligohydramnios and appears to be the major limiting factor aside from ductal constriction to long-term indomethacin therapy.