The purpose of the research presented in this thesis was to investigate the cell biology of rat spermatozoa. An additional aim was to utilise the knowledge obtained to aid the development of in vitro functional tests for the assessment of rat sperm fertility and identify potential markers of normal epididymal maturation. As mammalian spermatozoa migrate through the epididymis, they acquire the potential for fertilisation, characterised by the acquisition of the ability to express co-ordinated movement and the competence to undergo capacitance. The mechanisms by which epididymal maturation confers upon mammalian spermatozoa the potential to capacitate is poorly understood. These studies investigated the impact of epididymal maturation on the signal transduction pathways regulating tyrosine phosphorylation using the laboratory rat as an animal model, since this signal transduction pathway is thought to be central to the attainment of a capacitated state and expression of hyperactivated motility, both of which are prerequisites for fertilisation. Western Blot and immunocytochemical analysis demonstrated that epididymal maturation is associated with a progressive loss in phosphotyrosine expression located to the acrosomal domain. These differences in phosphotyrosine expression between caput and caudal epididymal spermatozoa appeared to reflect the normal in vivo situation. In addition, epididymal maturation of rat spermatozoa is also associated with an acquired competence to respond to high levels of intracellular cAMP by phoshorylating tyrosine residues on the sperm tail. Epididymal maturation also led to unique differences in the generation of reactive oxygen species (ROS) by spermatozoa obtained from the caput and caudal regions of the epididymis. Spermatozoa from both regions of the epididymis spontaneously generated equal levels of O₂ whereas only mature caudal spermatozoa generated significant levels of H₂O₂. In contrast, although both caput and caudal spermatozoa generated increased O₂^-. in response to NADPH, induced levels were significantly greater in the immature caput cells.