The tropospheric circulation pattern and its relation to rainfall in the Amazonas Basin are discussed. The general pattern of precipitation, during the summer, is closely linked to the position of an upper tropospheric high pressure system, which dominates the circulation oven. South America and whose center generality forms over Bolivia. This system weakens and moves northward during fall and winter while southern and eastern portions of the Amazonas basin experience a dry season, Variability in the rainfall pattern may be due partially to persistent convergence zones, originating from or associated with Southern Hemisphere frontal system, which greatly organize convection over tro_ pical Brasil. The sea breeze circulation, which affects the northern coast of Brazil, is an important factor, in organizing instability lines. At time, these lines propagate inland remaining active for up to 48 hours, in which time they cross the entire basin reaching the Andes. The spatial distribution of rainfall oven the basin may be linked to these instability lines.