Rat kidney microsomal fraction is able to catalyze the enzymatic desaturation of eicosatrienoic acid (20:3n-6) to arachidonic acid (20:4n-6) by the Δ5 desaturase pathway, in the presence of reduced nicotinamide adenine dinucleotide (NADH), adenosinetriphosphate (ATP) and coenzyme A (CoA). The substrate of the reaction [1-14C]eicosa-8,11,14trienoic acid (20:3n-6), was separated from the product [1-14C]eicosa-5,8,11,14-tetraenoic acid (20:4n-6) by reverse phase high-pressure liquid chromatography (RP-HPLC). These fatty acids were individually collected by monitoring the eluent at 205 nm and their radioactivity was measured by liquid scintillation counting. The Δ5 desaturase activity in kidney microsomes increased linearly with the substrate concentration up to 20 μM. Enzymatic activity was sensitive to pH with the maximum at 7.0 and was proportional with incubation time up to 10 min. The apparent Km and Vmax of Δ5 desaturase were 56 μM and 60 pmoles·min−1·mg−1microsomal protein, respectively. Neither the cytosolic renal fraction nor the cytosolic liver fraction enhanced the Δ5 desaturase activity. Contrary to a report but in accordance to others, the present results suggest that rat kidneys can synthesize arachidonic acid at least to satisfy partially their needs for eicosanoid