We have demonstrated that platelet activating factor (PAF) plays an important physiological role in the maintenance of high pulmonary vascular tone in fetal lambs, a role attributable to increased PAF receptor binding (J. Appl. Physiol. 85 (1998) 1079; Am J. Physiol. 278 (2000) H1168). In this study, we examined the possibility that increased PAF synthesis via de novo and remodeling pathways as well as decreased PAF catabolism in hypoxic state of fetal lungs may account for the PAF action in vivo. We investigated effect of oxygen tension on PAF synthesis by ovine fetal intrapulmonary venous (PV) and arterial (PA) smooth muscle cells pulsed with [3H]choline (de novo), or [3H]acetate (remodeling), while PAF catabolism was studied by assay of acetylhydrolase (PAF-Ah) activity. Hypoxia stimulated PAF synthesis by choline incorporation (pmol/10(6)cells) in both PVSMC (1.14+/-0.13 vs 0.53+/-0.05) and PASMC (0.39+/-0.12 vs 0.22+/-0.04). Hypoxia stimulated PAF synthesis via remodeling pathway only in PVSMC (408+/-32 vs 225+/-17) which was 5-fold greater than in PASMC (77+/-15 vs 105+/-24), however, with A23187 in remodeling pathway, PAF synthesis increased 5-fold compared to baseline conditions and then synthesis in hypoxia was greater than in normoxia in both cell types. Phospholipase A2 protein expression was significantly higher in hypoxia in both cells and was approximately 2-fold higher in PVSMC. PAF-Ah activity (nmol lyso-PAF/min/mg protein) was greater in hypoxia vs normoxia in PVSMC (0.81+/-0.24 vs 0.44+/-0.088), but in PASMC activity was less in hypoxia vs normoxia (1.68+/-0.24 vs 3.93+/-0.44). Compared to PVSMC PAF-Ah activity in PASMC was 4-fold higher in hypoxia. Our data demonstrate that (1) PAF synthesis in intrapulmonary SMC of fetal lambs occurs by both de novo studied by choline incorporation and remodeling pathways, the latter being predominant. (2) There is heterogeneity in PAF synthetic and catabolic activities in lung vasculature of fetal lambs. We conclude that increased PAF synthesis in veins by the two synthetic pathways coupled with decreased catabolism will result in a higher venous PAF levels in the hypoxic environment of fetal lungs. We speculate that in vivo, a high PAF level in veins will make more PAF available for binding to its receptors so as to sustain the desired high venous tone in the fetal pulmonary circulation.