At a gestational age of 27.9 days, pregnant rabbits were infused with enprofylline, a new xanthine with potent antiasthmatic effects. Following a priming dose of 5 mg/kg, 20 mg/kg were infused over a 2-h period. Controls received Ringer's solution. On completion of the infusion, the fetuses were delivered and the effect on their surfactant system was evaluated. Surface tension in airway fluid, measured with the pulsating bubble surfactometer, at minimal bubble size in the third cycle, was 21.8 +/- 2.2 (mean +/- SE) versus 28.7 +/- 1.7 mN m-1 (p less than 0.01). Pressure-volume loops of the lungs of the fetuses showed that at a deflation pressure 10 cm H2O the volume was 64 +/- 3 versus 49 +/- 3% (p less than 0.0005). The lung lavage fluid had a phospholipid phosphorus content of 41.9 +/- 3.8 versus 18.5 +/- 1.3 micrograms/g dry lung (p less than 0.0005). The values indicate that enprofylline caused a release of surfactant into the fetal airways.