Weanling rats were fed either a semisynthetic diet with no fat, with 28% by wt partially hydrogenated fish oil, or with 28% by wt arachis oil(control diet) for 6 or 7 1/2 months. The in vitro conversion of arachidonic acid to prostaglandin E2 by homogenates of the rat kidney medulla was measured by gaschromatography with electron capture detection. The kidney medulla of essential fatty acid deficient animals showed increased activity for the in vitro conversion of exogenous arachidonic acid to prostaglandin E2 when compared to the controls. The change of the enzymatic activity in the essential fatty acid deficient animals was reversible, as shown by refeeding. Inhibition of the prostaglandin synthetase was found at exogenous substrate concentrations higher than 50-100 muM.