The central dopaminergic system is implicated in the regulation of various physiological processes and behavioral responses, including social behavior. Although long-term individual housing of rodents is well known to alter their behavioral and neurochemical parameters, data interpretation remains ambiguous. In this work, we studied the effects of long-term social isolation on the behavior and state of the central dopaminergic system in male C57Bl/6 mice. The animals of the experimental group, aged 40–42 days, were housed singly in individual cages for six weeks, while the age-matched mice of the control group were housed in a group. Isolation did not affect locomotor and exploratory activity in the open field test compared to group housing. At the same time, singly-housed animals demonstrated a longer duration of social contacts in a resident–intruder model, as well as a weakened stereotypical behavior in the marble burying test compared to group-housed rats. These behavioral changes were accompanied by an increase in striatal mRNA levels of the genes encoding dopamine D1 and D2 receptors. Moreover, the level of the dopamine metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) decreased in the hypothalamus and increased in the frontal cortex in singly-housed compared to group-housed mice. The results provide a better insight into the effects of long-term social isolation on the behavior and dopaminergic system in mice.