The aim of the study was to assess the impact of social isolation of rats in the post-weaning period using behavioural tests aimed at assessing cognitive function, anxiety, and depressive -like behaviours. The monitoring was performed in male Wistar rats which were housed after weaning either individually (n = 8) or in pairs (n = 8) for 33 days. In the open field, rats kept in isolation reared less often (P < 0.05) than pair-housed rats. In the elevated plus-maze test, pair-housed rats entered the open arm more frequently (P = 0.002) and stayed in the closed arm less often (P = 0.019) compared to rats housed in isolation. In the forced swim test, climbing was seen more frequently (P = 0.016) in pair-housed rats whereas immobility was more common (P = 0.006) in rats housed individually. In the novel object recognition test, the pair-housed rats preferred (P = 0.014) the novel object whereas there was no difference (P = 0.107) in time spent by exploring familiar and novel objects in rats housed in isolation. Furthermore, juvenile rats housed for 33 days in isolation showed higher (P = 0.003) body weight gain during the monitored period than rats housed for the same period in pairs. Our findings are important not only in terms of assessing the impact of rat housing on their mental and physical development but also in terms of the accurate interpretation of the results of other experiments where the rat is used as a model organism.