The aim of the present study was to investigate gender differences in numbers and function of blood immune cells in stressed and non-stressed laboratory rats. Psychosocial stress in adult male or female rats was induced by social confrontation of an intruder rat with a resident opponent for 2 h. Behavioral analysis indicated that intruders of both sexes were clearly defeated and had markedly higher plasma corticosterone concentrations than unstressed home cage controls at the end of the confrontation. Lower numbers of CD4, CD8, and B cells as well as a reduced proliferative response of lymphocytes to ConA were observed in stressed groups of either sex. However, some important gender differences were also observed. Stressed males had higher granulocyte numbers than controls, while granulocyte numbers remained unchanged in stressed females. Similarly, stressed males had higher phagocytic activity than stressed females. Second, there was a gender difference in some basal values. Female controls had lower NK cell numbers than control males. Interestingly, NK numbers in stressed males decreased considerably, reaching the same low levels as in (stressed and control) females. In addition, females exhibited higher basal corticosterone concentrations than males. To summarise, these data do not indicate a superior blood cellular immune function in female rats, neither for the control nor the stress condition. However, the data clearly suggest that male and female rats should not be considered as a uniform group with respect to their immunological response to stress.