PURPOSE:To investigate allometric relationships among body mass (BM), muzzle-tail length (MTL), and tibia length (TL) in Wistar rats and establish their growth rate change parameters. METHODS:Eighteen male and 18 female Wistar rats were studied from the 3rd to the 21st week of age. BM, MTL, and TL were measured daily, and relative growth was compared using allometry. RESULTS:A positive correlation between BM and MTL (p<0.05) and BM and TL (p<0.05) was observed. Males and females showed comparable curves; however, females had turning points at a younger age. The allometric relationship between BM and MTL presented a regular increase until reaching a mass of 351 g (males) and 405 g (females). BM and TL showed an initial increase until 185 g (males)and 182 g (females), and then reached a plateau that finished at 412 g (males) and 334 g (females), to display another increase. CONCLUSIONS:The allometric relationship of body mass with animal length and tibia length was comparable for male and female rats, with female rats maturing earlier. Animal longitudinal growth occurred in a single stage. In contrast, tibia length depicted two stages of accelerated growth with an intermediate period of deceleration.