Resumo. The analysis of the oscillations of biomass stocks in tropical forests allows us to describe the behavior of these ecosystems in the face of natural or anthropogenic changes. The objective of this study was to evaluate the inventory and dynamics of biomass in the period (2007-2010) in dense rain forest in two areas of research in the National Forest - Tapajos, eastern Amazonia, called Area 1 and Area 2, 45 km distant from each other. The biomass was estimated using allometric equation and DAP, from 12 permanent plots, with six in each area. The results showed a significant difference in annual average stock biomass between Area 1 (293.19 ± 27.74, 298.21 ± 31.73, 299.60 ± 29.46, 298.11 ± 29.40 Mg. ha-1) and Area 2 (254.35 ± 69.61, 259.10 ± 70.05, 261.00 ± 69.43, 248.92 ± 61.78 Mg.ha-1). Besides that, it was noted annual change in stock biomass measurements at intervals of greater than one year due to fluctuations in mortality, recruitment and growth, besides the basal area and abundance of individuals in each area. The highest concentration of individuals occurred in diametric classes DAP 10 ≤ DAP ≤ 30 cm, however, it was verified that individuals with DAP ≥ 60 cm in lower concentration stored biomass equivalent to diametric classes 10 ≤ DAP ≤ 30 cm. We conclude that fluctuations in mortality, recruitment and growth, and abundance and basal area of these sites affected the stock biomass.