Burning of slashed tropical forests and pastures is among the most significant global sources of atmospheric emissions, yet the composition of the fuels and fires that creates these emissions is not well characterized. As part of the Smoke, Clouds, and Radiation-Brazil (SCAR-B) experiment, we measured total aboveground biomass (TAGB) as well as carbon, nitrogen, and sulfur pools in one cattle pasture and two slashed primary forests in Rondo⁁nia, Brazil. These pools were measured before and immediately after fires. From these data, we calculated the quantities of biomass and elements lost to the atmosphere during biomass burning. Prefire biomass in the pasture was 66 Mg ha-1; fire consumed 31% of this mass. Woody debris from the forest that occupied this site 12 years previously comprised 81% of the pasture prefire TAGB. Elemental inputs into the atmosphere (site losses) from the pasture fire were 9 Mg C ha-1, 88 kg N ha-1, and 5 kg S ha-1. Combining previous studies with this one, we calculate that the mean TAGB of Amazonian pastures is 74 Mg ha-1 with a mean combustion factor of 46%. Mean nutrient losses from pasture fires in Amazonia are 14 Mg C ha-1, 199 kg N ha-1, and 16 kg S ha-1. The TAGB of the two slashed primary forests before fire was 355 and 399 Mg ha-1 and following fire was 188 and 185 Mg ha-1 (i.e., a combustion factor of 47 and 54%), respectively. Combining this study with other studies of Amazon slashed primary forests, we calculate that the mean TAGB is 349 Mg ha-1 and the mean combustion factor is 48%. Total elemental losses arising from the primary forest slash fires in this study were notably higher than losses from the pasture site: 79 and 102 Mg C ha-1; 1019 and 1196 kg N ha-1; and 87 and 96 kg S ha-1. From this study combined with previous research in Rondo⁁nia and Par¿, we calculate that mean nutrient losses from primary forest slash fires are 88 Mg C ha-1, 1181 kg N ha-1, and 107 kg S ha-1. As rates of deforestation are remaining high in the Brazilian Amazon and pastures are the most frequent end product, it can be expected that these will remain the dominant sources of atmospheric emissions from Amazonia in the future. ¿ 1998 American Geophysical Union