In an effort to elucidate the functions of secreted phospholipase A2 (sPLA2) enzymes in vivo, we generated transgenic (Tg) mice for group V sPLA2 (sPLA2-V) and group X sPLA2 (sPLA2-X), which act potently on phosphatidylcholine in vitro. We found that sPLA2-V Tg mice died in the neonatal period because of respiratory failure. The lungs of sPLA2-V Tg mice exhibited atelectasis with thickened alveolar walls and narrow air spaces, accompanied by infiltration of macrophages and only modest changes in eicosanoid levels. This severe pulmonary defect in sPLA2-V Tg mice was attributable to marked reduction of the lung surfactant phospholipids, phosphatidylcholine and phosphatidylglycerol. Given that the expression of sPLA2-V is greatly elevated in human lungs with severe inflammation, our present results raise the intriguing possibility that this isozyme may contribute to ongoing surfactant hydrolysis often observed in the lungs of patients with respiratory distress syndrome. In contrast, sPLA2-X Tg neonates displayed minimal abnormality of the respiratory tract with normal alveolar architecture and surfactant composition. This unexpected result was likely because sPLA2-X protein existed as an inactive zymogen in most tissues. The active form of sPLA2-X was detected in tissues with inflammatory granulation in sPLA2-X Tg mice. These results suggest that sPLA2-X mostly remains inactive under physiological conditions and that its proteolytic activation occurs during inflammation or other as yet unidentified circumstances in vivo.