Microcalorimetric method was applied to study some Amazonian soils in order to establish the way by which the microbial soil activity is affected by the actual deforestation and burning suffered by the Amazonian rain forest. Different soil samples were collected in the Amazonian State of Brazil. Places with autochthonal vegetation and places that which had been deforested or burnt were elected due to its use in agriculture and cattle raising in an attempt to explore the difference in microbial growth behaviour. An LKB 2277 heat-how microcalorimeter was used for all measurements at 298.15+/-0.02 K. The microcalorimetric data are presented together with some other measurements such as the physicochemical parameters and the number of microorganisms in soil samples. Power-time curves recorded from soil samples amended with the same quantity of glucose showed significant differences among soils used in the study. Results obtained were systematically studied in a more quantitative way and the values of the microbial growth rate constant, mu, total thermal effect, Q(T), and duration of the peak-time, PT, were calculated from power-time curves. The collected data showed again important differences in the microbial activity among the soil samples suggesting that the native microbial activity of soils in tropical rain forests are dramatically affected by the deforestation and burning.