We examined the effects of three saturated fatty acids (myristic acid 14:0, palmitic acid 16:0, and stearic acid 18:0) on prostaglandin E 9-ketoreductase (PGE-9-KR, EC 1.1.1.189), which catalyzes the conversion of prostaglandin E2 (PGE2) into prostaglandin F2 alpha (PGF2 alpha). Palmitic acid inhibited PGE-9-KR activity dose-dependently, whereas the other two fatty acids had no effect. In spite of the structural similarity of these fatty acids, our findings suggest that, of the three, only palmitic acid has an inhibitory effect on PGE-9-KR.