We are maintaining two obese mutant rat strains (WNIN/Ob and WNIN/GR-Ob) in our animal facility since 1997. These rat colonies are perpetuated by crossing heterozygous littermates, since the obese phenotypes of both genders turned out be infertile. The present study revealed the reasons for this infertility. The male obese rats, though appeared normal in terms of sperm count, sperm motility and testis histology, however found wanting in terms of libido. This appeared to be due to low circulating testosterone levels seen in these animals, which should also account for low testis and accessory gland weights seen in them. The females exhibited delayed puberty, in terms of days taken for opening of vagina, irregular oestrus cycles and had small ovaries and short and stumpy uterine horns. The FSH peak observed in control lean animals during oestrus stage of the sexual cycle and also the E2 peak of normal oestrus cycle was conspicuously absent in these animals. They also showed elevated levels of progesterone throughout the sexual cycle. Thus the infertility seen in these mutants could be attributed to their abnormal gonadosteroid levels and the resulting anatomical and physiological defects.