Ontogenic studies aim to understand morphological variation of organisms throughout their lifecycle from an evolutionary and ecological framework. Akodon montensis is the most abundant terrestrial rodent species found in the Atlantic Forest (AF) of South America. Recently, Akodon paranaensis has been shown to also be extremely abundant in these forests. Identification between these species is difficult in the field. Understanding ontogenic variation can be valuable to understanding both evolutionary and ecological correlates which allow for the species to be so successful in the AF, and additionally what distinguishes this species from other sympatric species. We evaluated the skulls using traditional morphometrics for males and females in 6 age classes from juveniles to adults. A multivariate approach was implemented for 5 variables to identify the highest latency among age classes and gender. Discriminant Function Analysis was conducted to test significance between age groups and gender. We did not find evidence of sexual dimorphism, and age classes were easily discriminated. Our analysis revealed the highest allometric variation was related to the nasal passage and zygomatic arch. Given this preliminary data, we intend to compare these morphological trends with A. paranaensis in future studies. This information is valuable to improve our understanding of the ecological and evolutionary role of this species in the AF, which is a rapidly disappearing forest system.