Polyclonal antisera specific for prostaglandin endoperoxide (PGH) synthases-1 and -2 were used to determine the subcellular locations of each PGH synthase isozyme in detergent-permeabilized mouse 3T3 fibroblasts by indirect immunocytofluorescence. Antiserum to PGH synthase-1 demonstrated a mottled pattern of cytoplasmic and perinuclear staining of both serum-starved and serum-stimulated 3T3 cells. This pattern of staining is consistent with the results of earlier studies which demonstrated that PGH synthase-1 is associated with the endoplasmic reticulum and nuclear envelope of these cells. As expected, antibodies directed against a peptide unique to PGH synthase-2 failed to stain serum-starved cells, which lack appreciable levels of this second form of the enzyme. However, serum-stimulated 3T3 cells, which do express PGH synthase-2, showed the same pattern of staining with PGH synthase-2 antibodies as was observed with anti-PGH synthase-1 serum--mottled cytoplasmic staining and perinuclear staining. We conclude that the subcellular location of PGH synthase-2 is the same as PGH synthase-1 in murine 3T3 cells. Thus, the notable differences in the primary amino acid sequence--the signal peptide and the additional 18 amino acid C-terminal segment in PGH synthase-2--do not cause a change in localization. Colocalization of PGH synthases-1 and -2 implies that the source of arachidonate substrate, the site of PGH2 and prostanoid formation, and the mechanism of product transport from the inside to the outside of the cell are the same for these isozymes.