Terbutaline; a beta2-adrenergic agonist, and aminophyllin, a phosphodiesterase inhibitor, were given separately, or in combination, to rabbit fetuses on the 28th day of gestation. The possibility of these drugs causing a release of pulmonary surfactant was evaluated 3 h after i.m. injection directly to the fetus. Lung compliance was studied and lung lavage fluid assessed as regards content to total phospholipid phosphorus, lecithin/sphingomyelin (L/S) ratio, and surface activity. It was found that terbutaline, in a dose of 0.1 mg, had a significant (p less then 0.05) effect on each of the four parameters studied and, when given in a dose of 0.05 mg, significantly affected all except L/S ratio. Aminophyllin, in a dose of 10 mg, resulted in a 40% mortality, but in the surviving fetuses the drug had no significant (p greater than 0.05) effect on any of the four parameters. When 5 mg aminophyllin was given together with 0.05 mg terbutaline, the effect could be attributed to terbutaline.