We investigated the demographic parameters and the relationships between population density and extrinsic factors (temperature, rainfall, humidity and insolation) of two sigmodontine rodents ( Akodon montensis and Oligoryzomys nigripes ) in an Araucaria forest of southern Brazil. We followed the Cormarck-Jolly-Seber (CJS) method to estimate population parameters and density for each trapping session from November 2008 to August 2009. The relationships between density and extrinsic factors for each month were tested by simple linear regressions. Akodon montensis displayed the highest population size in three of the eight samples and showed no associations with any of the extrinsic factors analyzed. This species had higher estimated mortality than recruitment for all the seasons. Oligoryzomys nigripes presented higher population size in four of the eight samples, and density was associated with low temperatures and higher rainfall. The species showed similar estimated mortality and recruitment during the seasons, and the population size increased substantially during winter. The sex ratio was not biased in any of the species. In general, these rodents could be highly variable in time, and the observed abundance variations between seasons are in accordance with patterns described in other studies.