Mice account for over 80% of all animals used in experimentation. This study investigated how different housing conditions affected stress levels by measuring both corticosterone levels, using non-invasive faecal collection, and behaviour. Sixty outbred MF1 male mice were used which were separated into five different housing conditions at the beginning of the study, (A) individually housed, floor area 490 cm(2) per individual, (B) groups of three mice, floor area 163 cm(2) per individual, (C) groups of three mice, floor area 320 cm(2) per individual, (D) groups of six mice, floor area 160 cm(2) per individual, (E) groups of six mice, floor area 230 cm(2) with extra height per individual to allow visual contact. Mice in all housing conditions were provided with a basic enrichment of paper bedding and a plastic house. The results from this study showed that singly housed mice reduced their corticosterone levels over time after separation reaching a minimum from 14 days onwards. Groups of 6 mice housed together showed no difference over time. Also there was no significant difference in corticosterone levels between the different housing densities, with no differences for aggression or stereotypical behaviour suggesting that there is no ideal group density for this strain and sex of mouse. Providing additional enrichment to the cages caused a significant decrease in corticosterone levels for group housed mice, but individually housed mice remained unaffected by increasing their enrichment level. They spent significantly more time sleeping in the enhanced cage but without any reduction in stereotypical behaviour. For group housed mice, additional enrichment should be mandatory to reduce stress levels and therefore increase their welfare standards, while singly housed mice required only basic levels of enrichment and should be separated from their group for a minimum of 2 weeks before measurements are taken.