To study the effects of essential fatty acid deficiency upon rabbits, especially upon spermatogenesis, five immature, male, New Zealand rabbits were fed a purified diet devoid of fat for 14 weeks. The fatty acids of the testes showed a marked increase of 5,8,11-eicosatrienoic acid and a decrease in the members of the linoleate family of fatty acids. Gross evidence of essential fatty acid (EFA) deficiency included diminished growth and feed efficiency, and loss of hair. Total lipids, phospholipids, and free cholesterol of testes were found to be decreased, whereas triglycerides followed the reverse pattern. No qualitative or quantitative differences in the fatty acid composition of seminal vesicles were found in the deficient and control groups. Testes of deficient animals showed an extensive degenerative change in the seminiferous tubules; no stage beyond secondary spermatocyte was evident. Glucose 6-phosphate dehydrogenase and Δ5-3β-hydroxysteroid dehydrogenase activity of Leydig cells in both groups showed that these enzymes were present. Male accessory gland weights were significantly reduced in the deficient animals, indicating reduced androgen secretion. Histological examination of the anterior pituitary gland showed signs of degenerative changes in PAS-positive basophilic cells, which appeared to be shrunken and partially degranulated in EFA-deficient rabbits, suggesting that EFA deficiency is accompanied by diminished secretion of the anterior pituitary. There was also an increase in the number of chromophobes and a decrease in acidophil cells. These observations suggest that the degeneration of gonads observed during essential fatty acid deficiency may be due to primary impairment of anterior hypophyseal function.