Heterophil to lymphocyte ratio and duration of tonic immobility are indicators of stress and fearfulness, respectively. This study was designed to examine the effects of the male to female mating ratio on the heterophil to lymphocyte ratio and on the duration of tonic immobility in two different experiments. In the first experiment, four different mating ratios (1:11, 1:5, 1:3, and 1:1) and 10 different Spanish breeds of chickens were allocated to breeding pens of 12 birds each. Experiment two used two different mating ratios (1:11 and 1:1) and four breeds allocated to pens of 60. There were significant differences among mating ratios for the heterophil to lymphocyte ratio in females of both group sizes. Hens housed with a mating ratio of 1:1 had a higher heterophil to lymphocyte ratio than hens with a 1:11 mating ratio in the group with 12 or 60 birds. In the 1:1 mating ratio, hens had significant heterophilia and lymphopenia. The 1:5 and 1:3 ratios did not differ from the 1:11 ratio. In the 1:1 ratio, the effect of group size was significant, but it was not significant in the 1:11 ratio. Tonic immobility did not differ between the mating ratios for hens. In the mating ratio of 1:1 and in the group size of 60 birds, cocks showed a shorter duration of tonic immobility than cocks housed with a 1:11 mating ratio. Results in the study suggest that very high mating ratios should not be used in conservation populations because they increase physiological and psychological stress responses.