During July and August 1985, the sources of methane and its flux to the troposphere were measured from a houseboat laboratory anchored in Lago Calado, a stratified, dendritic lake of about 6 km2 area, located in the central Amazon basin. Methane concentrations in the mixed layer of the lake were varied (0.0001--0.0055 mM) and usually less than 0.004 mM CH4, with no consistent temporal trend. Methane concentrations increased with depth across the thermocline as oxygen dropped to less than 0.1 mg O2 L-1. Over 6 weeks, methane increased from less than 0.08 to greater than 0.21 mM in the anoxic hypolimnion below 6 m. Methane in the pore water approached saturation, with a pure methane atmosphere within 5 cm of the sediment/water interface. The gradient-supported flux from the sediments to the overylying water could account for the methane increase in the bottom waters plus the surface flux. The measured methane flux from the surface of the open lake to the atmosphere averaged 27 mg CH4 m-2 d-1. This was consistent with the buildup in ambient methane in the nocturnal surface mixed layer of the troposphere. Ebullition contributed 70% to the average total flux. The diffusive flux measured with a static chamber ranged from 0 to 34 mg CH4 m-2 d-1, with an average of 8.3 mg m-2 d-1. From July 20 to September 2, 1985, average ambient air concentration was 1.89 (¿0.16) ppm CH4, with a diurnal variation of 0.27 ppm. ¿ American Geophysical Union 1988