Rabbits delivered at 27.0 days of gestation were studied after administration of cortisol (2 mg/kg/day), aminophylline (6.25 mg/kg/day), or sterile saline to the does on days 24–26 of gestation. Survival at 60 min was 52.9% in the aminophylline-treated group and 22.2% in the control and cortisol-treated groups with all animals being in a warm, oxygen-enriched environment and receiving frequent tactile stimulation. Lung volume at 30 cm H2O was lower in the cortisol-treated group than in the controls or aminophylline-treated group in animals surviving for 60 min (Table 2). The aminophylline-treated group retained significantly more gas at low pressures on the deflation curve (Table 2) and had significantly more phosphati-dylcholine recovered in lung lavage fluid (Table 3) than the other groups. Aminophylline appears to have enhanced lung maturation better than cortisol in this experimental model.SpeculationImproved survival of prematurely delivered rabbits after aminophylline administration (as compared with cortisol) may be due to a combination of factors including enhanced maturation of the lungs as well as stimulation of the respiratory center.