Studies were conducted to determine the efficacy of a dietary technique for reducing prostaglandin (PG) synthesis in adult rats. Rats weighing 280-318 g were fed either essential fatty acid (EFA)-deficient or EFA-adequate diets for 10-17 days after a period of food deprivation. Synthesis of renal papillary PGE 2 and aortic PGI2 from endogenous precursor in vitro were estimated by liquid chromatographic and bioassay/radioimmunoassay techniques, respectively, as indices of the capacity of the technique to induce EFA deficiency. PGE synthesis and PGI2 synthesis by isolated tissues from rats fed the EFA-deficient diet were significantly decreased (ca. 50%) relative to control rats fed an EFA-adequate diet. Body and renal papillary weights were not significantly altered by the EFA-deficient diet.