To test the hypothesis that fetal lung maturation can be accelerated by one of the xanthine derivatives, aminophyllin was given to 40 pregnant rabbits beginning on the 20th gestational day for a period of 7-10 days. The fetuses were delivered by cesarean section and fetal lung maturity was assessed by determining the biochemical, functional, and ultrastructural characteristics of aminophyllin-treated vs. control animals. The phospholipid content of the lung tissue homogenate from the aminophyllin-treated group was significantly higher than in the control subjects (saline injected) at 28 days of gestation (421 +/- 9 vs. 368 +/- 12 mug/mg wet wt, mean +/- SEM) and at 29 days of gestation (531 +/- 10 vs. 475 +/- 20). The alveolar wash phospholipid content of the aminophyllin-treated group was higher at 30 days (167 +/- 9 mug/mg dry wt, mean +/- SEM vs. 117 +/- 17). The lung compliance derived from pressure volume curves was also significantly higher in the aminophyllin-treated group when compared with controls at 27 days of gestation (0.023 +/- 0.0005 ml/cm H2O, mean +/- SEM vs 0.010 +/- 0.0002) and at 28 days of gestation (0.048 +/- 0.0003 vs 0.035 +/- 0.0006). There was no significant difference in the number of lamellar bodies in the type II cells between the aminophyllin-treated and the control groups. The data show that aminophyllin has accelerating effects on fetal lung maturation in rabbits when the drug is given to pregnant rabbits during the last 7-10 days of gestation.