Prostaglandin endoperoxide H synthases-1 and -2 (PGHS-1 and -2) are integral membrane proteins associated with the luminal surface of the endoplasmic reticulum (ER) and the nuclear envelope (NE). The C-terminal sequences of PGHSs end in -Ser/Pro-Thr-Glu-Leu (-S/PTEL), a sequence similar to one of the known ER targeting sequences, -Lys-Asp-Glu-Leu (-KDEL). Previous immunofluorescence studies from our own and another laboratory had indicated that both native ovine (o) PGHS-1 and oPGHS-1 mutants with modified or deleted -PTEL sequences were localized primarily in the ER when the proteins were expressed for 24-40 h following transient transfection of cos-1 cells. However, in characterizing an oPGHS-1 mutant lacking 15 amino acids from the C-terminus (deltaCT15 oPGHS-1), we found that when this mutant was expressed for a shorter period (18 h) following transfection, the enzyme was concentrated near the nucleus in what appeared to be the Golgi apparatus; similar results were observed when the -P/STEL mutants of oPGHS-1 prepared previously (i.e., deltaPTEL, L600N, and L600R oPGHS-1) were retested using the 18-h posttransfection expression time; in contrast, under the same conditions, the native enzyme was localized throughout the cytoplasm (i.e., in the ER). When the cos-1 cells transfected with each of the various C-terminal mutants were treated with Brefeldin A, the immunofluorescent staining was redistributed to the ER, whereas the distribution of native oPGHS-1 was unaffected. A human PGHS-2 (hPGHS-2) mutant in which the C-terminal leucine residue was replaced with an asparagine (hPGHS-2 L604N) was also found to localize to the Golgi apparatus following 18 h expression in transfected cos-1 cells. These results establish that the C-terminal-S/PTEL sequences of both PGHS-1 and PGHS-2 target the enzymes to the ER. PGHS-2 is more highly concentrated in the NE than in the ER, but apparently this isozyme traverses the Golgi apparatus and returns to the ER prior to its becoming concentrated in the NE.