Individual hormone profiles can be important generators of phenotypic variation. Despite this, work on the consequences of hormone profiles has traditionally ignored inter-individual variation within natural populations. A key advance is to examine repeatability in hormone profiles and their links to behavioral traits under selection. We firstly show that individuals within a free-ranging population of the Australian lizard Egernia whitii exhibit temporal repeatability in their testosterone concentrations as well as their aggressive responses towards conspecific intruders. There were significant, sex-specific links between testosterone and aggression; testosterone and aggression was negatively linked in males, while there is no relationship in females. As conspecific aggression has significant consequences for fitness-related traits (parental care, mating strategies) in this species, inter-individual variation in testosterone concentrations, through their effects on aggression, could have important implications for individual fitness. In the second part of this study, we conducted two complementary experiments designed to investigate the activational relationship between testosterone and aggression in male lizards. First, we investigated whether a conspecific aggressive challenge induced a testosterone response and second, we artificially manipulated testosterone concentrations to examine whether this changed aggression levels. Testosterone concentrations were not influenced by an aggression challenge. However, there was a slight indication that receiving a challenge may influence intra-individual consistency of testosterone, with individuals not receiving an aggression challenge maintaining consistency in their circulating testosterone concentrations, while those individuals that received a challenge did not. Our study adds increasing evidence that the relationship between testosterone and aggression is not straightforward, and promotes the investigation of alternative hormonal pathways and differences in neuro-synthesis and neuroendocrine pathways to account for species variable testosterone – aggression links.