Plasma oestradiol-17 beta and testosterone concentrations as possible causes of the infertility of congenitally obese Zucker rats.

The plasma oestradiol-17 beta concentrations of obese and non-obese female Zucker rats have been measured in three phases of the oestrous cycle. The oestradiol concentrations of both phenotypes were similar, and changed normally with the oestrous cycle. The weights of the uteri also changed normally with the cycle. Plasma androgen concentrations in male Zucker rats have also been measured: the mean concentration was slightly but significantly lower in obese rats, and androgen-sensitive tissues were slightly reduced in weight. The oestradiol-17 beta concentrations in males of both phenotypes were similar. It seems unlikely that deficient plasma concentrations of gonadal hormones cause the infertility of obese rats of either sex.