The cycle of atmospheric CO2 in the Amazon Basin was studied using measured vertical profiles of CO2 concentrations in the canopy and aloft and direct measurements of CO2 emissions from soils. The results provide detailed information on daily exchanges of air between the tropical forest (0--30 m) and the atmospheric boundary layer (30--2000 m). The forest was a net source of CO2 at night and a sink during the day. High concentrations of CO2 (380--400 ppm) accumulated in the forest at night and were transported into the nascent mixed layer in the morning. Between 1000 and 1200 LT, CO2 in the mixed layer declined to concentrations 4--6 ppm lower than free tropospheric values and remained low for the rest of the afternoon. The afternoon mixed layer became isolated at night, and the low values of CO2 were preserved until a new mixed layer developed the next morning. Entrainment of air into the mixed layer appeared to be a one-way process during the early stages of mixed layer growth. The rate for uptake of atmospheric CO2 by forests (soil and canopy) near midday was estimated to be 9(¿4) kg C ha-1, and mean uptake over the sunlit period was about 2.8 (¿1.2) kg C ha-1 hr-1. Forest soil emitted CO2 at a mean rate of 1.8 (¿0.2) kg C ha-1. The atmospheric CO2 cycle over wetlands was weaker than over forests, with daytime uptake rates of approximately 1.6 kg C ha-1 hr-1. Rivers remained net sources of CO2 throughout the day. ¿ American Geophysical Union 1988