Diabetes Mellitus is chronic systemic disease associated with characteristic long-term complications that affect a wide range of cells, tissues and organs. As such, diabetes has the potential to affect male fertility at a number of levels. Despite this, the prevalence of reproductive disturbances in diabetic men remains largely unknown. This study was carried out to evaluate the effects of diabetes on functional, endocrine, cellular and molecular aspects of male reproduction. Sexual function, assessed using a validated multi-dimensional questionnaire, and the reproductive endocrine axis were compared in diabetic and nondiabetic subjects. This study revealed a high prevalence of erectile dysfunction in diabetic compared to control subjects. Although the reproductive endocrine axis was largely normal, bioavailable testosterone levels were reduced in diabetic subjects. This may contribute to diabetes- associated sexual dysfunction and impair androgen-dependent elements of male reproduction. Sperm from diabetic and non-diabetic men were examined using conventional and molecular techniques. Sperm nuclear DNA was assessed using the alkaline COMET assay and mitochondrial DNA by Long- Polymerase Chain Reaction. Although conventional semen profiles were similar, sperm nuclear and mitochondrial DNA damage was significantly higher in diabetic men. This suggests that exposure to the diabetic environment is associated with damage to sperm at a molecular but not a cellular level. Subsequently, oxidative DNA damage was assessed using High Performance Liquid Chromatography to evaluate levels of the oxidative DNA adduct 7,8-dihydro-8-oxo-2'-deoxoguanosine (8-OHdG). Significantly higher levels of 8-OHdG were found in sperm from diabetic compared to non-diabetic men, suggesting that oxidative stress is an important factor in diabetes-asociated sperm DNA damage. Sustained exposure to supra-physiological levels of glucose can result in the formation of Advanced Glycation End-Products (AGEs). These can damage cellular proteins and DNA directly or cause increased oxidative stress via interaction with their receptor, RAGE. The presence, localisation and relative quantitation of AGE and RAGE were evaluated in the testis, epididymis and sperm using immunohistochemistry, immunocytochemistry and an Enzyme Linked Immunoabsorbant Assay. The AGE - N-e-Carboxymethyl-lysine (CML) and RAGE were found throughout the testis and epididymis. In sperm a characteristic pattern of staining was observed on the sperm head and equatorial region. Although less CML was found in sperm and seminal plasma of diabetic men, significantly higher levels of RAGE were present. These findings suggest that AGE/RAGE may be important in diabetes- associated reproductive disturbances. Finally, differences in gene expression profiles between diabetic and nondiabetic men were evaluated using a microarray platform. A number of genes were found to have significantly altered expression compared to controls. Amongst these, several are linked with DNA repair, oxidative stress and glycation. This supports the results of earlier chapters that demonstrated increased oxidative sperm DNA damage in diabetic men. Moreover, it provides indirect evidence supporting a role for AGEs and RAGE in the pathogenesis of such disturbances.