Significant difference (P < 0.05) between values in the presence and absence of exogenous vitamin E.membrane. This view is supported by reports that the fertilizing ability of chicken semen is improved by dietary supplementation with n-3 fatty acids (Blesbois et al., 1997a,b; Kelso et al., 1997b).The effects of the various dietary oils on the recipient birds are not confined to changes in the fatty acid composition of the tissues (Surai et al., 1999) and semen (current study). Feeding birds with the tuna orbital oil diet without supplementation with excess vitamin E resulted in a marked depletion of vitamin E from the tissues. Presumably this depletion is due to the increased peroxidative susceptibility of tissues enriched with the highly polyunsaturated 22:6n-3, which places excessive demands on the antioxidant capacity of the bird. It is also evident that the antioxidant capacity of the semen can be compromised by the highly polyunsaturated dietary oils, as indicated by the reduction in vitamin E concentration in semen of the birds fed the tuna orbital oil (low vitamin E) diet. Moreover, the susceptibility of semen to lipid peroxidation in vitro was increased as a result of feeding the oils rich in 20:4n-6 or 22:6n-3 with the lower level of vitamin E, an effect that was effectively prevented by the inclusion of supplementary vitamin E in the diet. However, the dietary effects on the peroxidative susceptibility of the semen did not produce significant differences in fertilizing ability as assessed by artificial insemination.Among the multifarious consequences of feeding chickens with oils that provide diverse polyunsaturate profiles, the generation of increased concentrations and disparate compositions of eicosanoids may be relevant to the changes in sperm output. Arasco oil provides large amounts of 20:4n-6 for potential conversion to type 2 prostaglandins and type 4 leukotrienes, whereas tuna orbital oil provides substrate for the synthesis of type 3 prostaglandins and type 5 leukotrienes after the retroconversion of 22:6n-3 to 20:5n-3 (Lands, 1979; Fischer, 1989; Smith, 1989; Sardesai, 1992). Although prostaglandins in seminal plasma regulate various aspects of sperm function (Gottlieb and Bygdeman, 1988), their relevance to the present findings is unclear, especially since a range of prostaglandins has been shown to inhibit spermatogenesis in rodents (Abbatiello et al., 1976). Other possible mechanisms whereby dietary fatty acids could promote spermatogenesis, such as the regulation of gene expression (Jump and Clarke, 1999), remain to be examined. An important consideration is the potential interaction of polyunsaturated fatty acids or their derived eicosanoids with the hypothalamo-pituitary-gonadal axis and the hormonal control of spermatogenesis (De Kretser, 1990; Etches, 1996). Thus, the effects of dietary polyunsaturates on the secretion of GnRH, LH, FSH and testosterone, and on the responsiveness of the relevant types of cells to these hormones, may be worthy of investigation.Irrespective of the unde...