Obesity seems affect reproductive function, lowering sex hormones and reducing sperm concentration. Excess of lipids on diet influences metabolism and testis reproductive function. Among physiological determinants of weight and appetite are Leptin and insulin, hormones secreted in proportion to fat mass and glucose levels regulating peripherally the metabolism, moreover in hypothalamus interact with receptors, favoring satiety (1). Obese people may have higher serum concentrations of this hormones and exhibit resistance to its action. The relationship between obesity, reproductive changes and metabolic syndrome is not yet fully clarified (2). Thus, the aim of the study was evaluate the effect of high fat diet, containing soybean oil, on body composition and reproductive system of young male rats. Wistar rats, at 21days (after weaning) were fed with diet containing 7% (control diet, C) or 19% (high fat diet, HF) of soybean oil, manufactured according to AIN-93 G, until 30 and 60 days of age. At the end of each experimental period, body composition was evaluated by Dual-energy X-ray Absorptiometry (DEXA) and liver, adipose tissue, testis, epididymis and blood were collected. Serum analysis of glucose, lipids, insulin, leptin and testosterone were measured. During all experimental period food intake and body mass were monitored but no difference was observed between groups. After feeding animals with a high-fat diet for nine days showed a higher testosterone concentration (C:1.36±1.1; HF:24.9±6.4ng/dl), while surprisingly lean mass (C:60.0±3,03; HF: 42,6 ±3,47g) and triglycerides (C:60.0±9.6; HF:32.7±6.9mg/dl) decreased, in spite of the protein content control and the high percentage of soybean oil in the diet. At 60 days, testosterone (C: 0.95±0.26; HF:30.4±3.5ng/dl), epididimal mass (C:0.3±0.01; HF:0.41±0.026g), mesenteric adipose tissue mass (C:2.15±0.18; HF:3.09±0.32g), liver mass (C:8.25±0,2; HF: 10.5±0,55g) and glucose (C:167.5±10.8; HF: 269.8±20.0mg/dl) increased in HF group. The other evaluated parameters, total fat mass (C:24.8±2,0/HF:28.0±2.17g;C:64,0±1,54/HF:59,67±6,108g); leptin (C: 0.98±0.17/ HF:1.34±0,17;C: 1,50±0,08/HF:2,04±0,35 ng/ml) and insulin (C: 21.9±29/HF: 21.4±3.9; C:25.8±4.6/HF:27.8±4.6 ng/ml) did not differ between the groups at 30 and 60 days, respectively. Since nine days of high-fat diet intake, the excess of lipids reflects negatively on testis function with high levels of testosterone. Moreover, after thirty nine days of high-fat diet intake the effects on adiposity, glycemia and reproductive function are more evidents. It could predict some adult reproductive dysfunction, once the changes started early and were increasing in these young animals.