Female rats were weaned and fed a semipurified diet lacking in essential fatty acids. After 160 days, the deficient diet was supplemented with varying amounts of gamma-linolenic acid. Changes in body weight and feed efficiency were measured. Total liver phospholipid fatty acids were also analyzed. Supplementation with gamma-linolenic acid to the deficient diet for 7 days led to improvements in body weight and feed efficiency of the deficient rats. The liver phospholipid fatty acid composition returned to a normal pattern. There was a reduction of 5,8,11-eicosatrienoic acid and an increase in the arachidonic acid. Thus, there was a fall in the triene: tetraene ratio with increasing dietary supplementation of gamma-linolenic acid. The essential fatty acid potency, the minimum dietary requirement for this fatty acid, and the widely accepted levels of the minimum requirements of dietary essential fatty acids are discussed.