The relationship between alveolar saturated phosphatldylchollne pool size and compllance was evaluated in control and maternal corticosteroid-treated 27-, 28-, and 29~ ay gestational age pretenn ventilated rabbits. Surfactant pool sizes below about O. 71J. 1Tlol saturated phosphatldylcholine/kg body weight did not alter compliance In either group. Compliance increased to values comparable to surfactant-treated or term newborn rabbits over a narrow range of saturated phosphatidylchollne pool sizes from about 0.7 to 1.2 Ilmollkg for corticosteroid-treated rabbits and from about 0.9 to about 2.3 Ilmollkg for control rabbits (p< 0.01). When these results are compared with those from surfactant-treated rabbits, almost an order of magnitude less endogenous surfactant than exogenous surfactant Is needed to alter compliance in pretenn ventilated rabbits.