Haem oxygenases produce carbon monoxide, which, like nitric oxide, is a gaseous messenger molecule that is one of several important survival factors in ovarian follicles. However, little is known about the expression and possible functions of these enzymes in granulosa cells. The purpose of this study was to investigate the expression and possible role of haem oxygenases in porcine granulosa cells (PGCs). We obtained frozen sections of porcine ovaries and PGCs from ovarian follicles of various sizes by needle aspiration, and examined the expression of haem oxygenase-1 (HO-1; inducible type) and HO-2 (constitutive type) in PGCs by immunohistochemistry, RT-PCR, western blotting and flow cytometry. Both types of haem oxygenase were identified in PGCs throughout follicular development, but HO-1 was expressed primarily in granulosa cells in atretic follicles. We also investigated the effect of haem oxygenases on apoptosis of granulosa cells (flow cytometry to detect subdiploid DNA fluorescence) and on expression of Fas ligand (quantitative analysis of western blotting and flow cytometry). In tightly bound PGCs, the mean proportion of apoptotic cells treated with 1 microM haemin (a haem oxygenase substrate) was approximately 1.7-fold greater than that in untreated controls, and zinc protoporphyrin IX (ZnPP IX; a haem oxygenase inhibitor) completely inhibited the increase in apoptosis induced by haemin in 24-h culture. Conversely, in weakly associated PGCs, the proportion of apoptotic cells was not altered by haemin. The quantity of Fas ligand protein was increased in a dose-dependent manner in tightly bound PGCs treated with haemin compared with controls, and the haemin-induced increase in Fas ligand protein was inhibited by ZnPP IX. Thus we identified inducible HO-1 and constitutive HO-2 in PGCs throughout follicular development, and we conclude that products of reactions catalysed by haem oxygenases are likely to be important autocrine/paracrine factors that regulate apoptosis in PGCs.