In two experiments a total of twelve male rats were reared from weaning for up to 63 weeks on an essential fatty acid (EFA)-deficient diet alone (2 X two animals) or supplemented with the methyl esters of linoleic acid (18:2 omega 6) (2 X two animals) or linolenic acid (18:3 omega 3) (2 X two animals). Testicular development was normal in rats given 18:2 omega 6, but in rats fed the EFA-deficient diet alone, and in those supplemented with 18:3 omega 3 the testes were reduced in size. Histologically, a degeneration of the seminiferous tubules was noted, with progressive loss of the germinal cells, and with an absence of spermatozoa in the lumina of the seminiferous tubules and epididymides. Leydig cells appeared unaffected, and were prominent. The six rats in Experiment 1 were capable of mating with females reared on commercial diets, but only the two 18:2 omega 6 supplemented animals were fertile. There was a marked reduction in the percentage of arachidonic acid (20:4 omega 6) and docosapentaenoic acid (22:5 omega 6) in the total fatty acids of the atrophic testes. There was no compensatory increase in long-chain derivatives of 18:3 omega 3 in the 18:3 omega 3 fed rats and it is concluded that linolenic acid cannot replace linoleic acid in the development of the rat testis.