Abstract: Objective To investigate the relationship between the body weight change and the mRNA or protein expression of uncoupling proteins (UCPs) in OLETF rats. Methods Body weights were measured per weeks. UCP1 protein expression in the brown adipose tissue was detected by western blotting, while UCP2, UCP3 mRNA expression in the skeletal muscle was measured by northern hybridization in the OLETF and LETO rats at age of 7,10 and 25 weeks. Results OLETF rats gained weight at a faster rate than LETO rats, and the difference in the rate of weight gain was most prominent around 9 weeks of age. UCP1 protein level was 1.9-fold higher in LETO than OLETF rats at 10 weeks of age (p<0.05). UCP3 mRNA level was 5.5-fold higher in LETO than OLETF rats at 10 weeks of age (p<0.01). Conclusions: UCP1 and UCP3 expression may play an important role particularly in the early stage of the development of obesity in OLETF rats.