Oxytocin (OT), locally synthesized in the testis, is involved in androgen biosynthesis. The use of polyunsaturated fatty acids (e.g., fish oil) in the diet may improve the fertilizing ability in mammals. Cystinyl aminopeptidase (oxytocinase) activity plays a major role regulating the functional status of OT. Sex steroids and the type of the fatty acid used in the diet modify aminopeptidase activities in serum. In the present study, the authors compared the effect of a fish oil supplemented diet with two other diets supplemented with saturated oils (lard and coconut) on oxytocinase activity in the testis of mice. The enzymatic activity was determined fluorometrically using cystinyl-beta-naphthylamide as substrate. The results demonstrated higher levels of oxytocinase activity in mice fed the diet supplemented with fish oil than in those that were fed diets containing lard or coconut oils. The testicular functions in which OT is involved may be attenuated by the use of fish oil in the diet.