Abstract: Objective To determine whether there are different in obesity-prone and obesity-resistant rats before diet-induced obesity occurs.Methods Male Sprague-dawley rats were divided into control group and high-fat group randomly.After initial blood sampling,rats were fed a control diet or a high-fat diet for 7 weeks.Over the 7-wk period,rats fed the high-fat diet diverged into two groups based on body weight gain.Serum testosterone and estradiol concentrations were determined in rats befor exposure to the high-fat diet by radio immunoassay.Results The obesit-yprone group had a low testosterone concentration before the onset of obesity(0.50±0.46nmol/L)compared with obesity-resistant rats(1.95±1.70nmol/L)and control rats(1.03±0.47nmol/L).Estradiol levels were not different between the groups.Conclusion Low testosterone secretion exists before dietary challenge or before onset of obesity and may contribute to the susceptibility to obesity observed in these rats.