The purpose of this study is to establish reference data on several hematological parameters in the cotton rat. Eight hematological values were measured in 28 males and 29 females at 16 to 18 weeks of age. Females showed significantly lower RBC, Ht and MCV values than males. Female Hb and PLT values tended to be lower than male values, although differences did not reach a conventional level of significance. WBC and PLT values varied widely in both sexes.