Prostaglandin (PG) E synthase was found to be widely distributed in the microsomal fractions of rat organs. Among them, an extremely high activity was seen in the deferent duct (112 nmol/min x mg) and other genital accessory organs (10-20 nmol/min x mg). In non-genital organs, the kidney had the highest activity (8 nmol/min x mg). Most of the PGE synthase activity in these organs required glutathione (GSH). In contrast, the enzyme activity in the heart, spleen, and uterine microsomes did not require GSH for its catalytic activity. In view of these data and those of other enzymatic parameters (Km values for PGH2 or pH optima), we suggest that two different types of PGE synthases, GSH-dependent and GSH-independent enzymes, are present in microsomal fractions of rat tissues.