Spermatozoa are highly specialised cells which display a range of unique features associated with their crucial function of egg fertilisation. One of the most striking characteristics of spermatozoa, in biochemical terms, is the extremely high proportion of long chain highly polyunsaturated fatty acids present as components of the plasma membrane phospholipids. This high degree of unsaturation is almost unique amongst animal cells; the only other cell types which display similar levels of these polyunsaturates are the neurons of the brain and retina. The reason why spermatozoa exhibit such an unusual fatty acid composition is not clear but it is feasible that the highly unsaturated phospholipids may confer a high degree of flexibility on the sperm plasma membrane as well as provide a potential energy source in order to facilitate the characteristic flagellar motion of these cells. There is also evidence that spermatozoa lipids play a crucial role in the membrane fusion and signal transduction events associated with the acrosome reaction and fertilisation. Initial observations were made between the semen of domestic cockerel and bull with respect to animal ageing and semen quality. Within both species similar patterns were observed in that with age there was a loss of the long chain C20 and C22 polyunsaturated fatty acids accompanied by a loss of the major antioxidant enzyme systems. A decrease in phosphatidyl ethanolamine and an increase of phosphatidyl choline with age were also observed to be associated with a reduction in semen quality parameters and fertility in both species. In contrast cockerels displayed significant increases in spermatozoa and seminal plasma lipid levels where the bull exhibited losses, reflecting possible differences in spermatozoa metabolism and function.