Recent studies have suggested that the early perinatal environment is very important to health status in the adult years. We have tested the hypothesis that the perinatal ambient temperature alters sympathetic activity during early life and affects body composition in adult life. We have also hypothesized that the effect of ambient temperature might differ between strains of rat that were resistant or susceptible to dietary obesity. In this set of experiments, Osborne-Mendel (OM) rats that become obese eating a high fat diet were compared with S5B/Pl (S5B) rats that do not become obese eating a high fat diet. From one week before birth, rat litters were raised at either 18°C or 30°C for the first two months of their life while consuming a chow diet after weaning at three weeks of age. At two months of age they were housed at normal housing temperature (22°C) and the diet was changed to provide either a high fat (56% energy)diet or a low fat (10% energy) diet, both of which were equicaloric in protein. Until 6 weeks of age the OM rats reared at 30°C weighed more than the OM rats raised at 18°C. However, once transferred to the special diets the OM rats reared at 18°C gained more weight than those reared at 30°C although all animals were now maintained at the same ambient (22°C) environmental temperature. Perinatal temperature had no effect on body weight gain of the S5B rats on either diet. The OM and S5B rats reared at 18°C also had higher intakes of the high fat diet than those reared at 30°C when measured at 12 weeks of age. Further, the increase in metabolic rate in response to the 3 agonist, CL 316,243, was greater in both OM and S5B rats reared at 18°C than in those reared at 30°C. Though not significant, this temperature dependent difference in response appeared to be attenuated in rats fed the high fat diet. In contrast, beta-3 adrenergic receptor and uncoupling protein-1 mRNA levels were higher in the rats of both strains reared at 30°C. Perinatal temperature during early growth differentially affects body weight in OM and S5B rats while having similar effects on food intake, response to a ₃ agonist and increased expression of BAT 3AR and UCP1. These results do not directly explain the differential effect on body weight gain. In conclusion, the absence of any differences in body weight phenotype in S5B rats with perinatal temperatures suggests that this rat may be better able to regulate growth and energy balance despite early environmental influences on epigenetic programming. This demonstrates that genetics can alter the effects of perinatal environmental temperature on postnatal body weight and response to a high fat diet.