With an area of 872 000 km2, the Bolivian Amazon basin corresponds to the upper part of the basin of the River Madera, the principal southern tributary of the Amazon which has a mean discharge of 17 000 m3 s-1. The Madera River receives 40 x 106 t year-1 of dissolved material from its two principal tributaries (the Mamore and the Beni) (1983-1989 average). The basins of these rivers include very contrasting biogeographical environments, from the glaciers of the eastern mountain chain of the Andes (6500 m a.m.s.l.), to the humid tropical rainforest of the Amazon plain (150 m a.m.s.l.). This work, based on the sampling of 395 rivers of the Amazonian basin of Bolivia, describes the major features of the regional distribution of the water chemistry (major elements of the dissolved phase) relative to the geology of the basin.