Abstract1 Carbon dioxide and water vapour fluxes were measured for 55 days by eddy covariance over an undisturbed tropical rain forest in Rondonia, Brazil. Profiles of CO2inside the canopy were also measured.2 During the night, CO2concentration frequently built up to 500 ppm throughout the canopy as a result of low rates of exchange with the atmosphere. In the early morning hours, ventilation of the canopy occurred.3 Ecosystem gas exchange was calculated from a knowledge of fluxes above the canopy and changes of CO2stored inside the canopy. Typically, uptake by the canopy was 15 μmol m−2s−1in bright sunlight and dark respiration was 6‐7 μmol m−2s−1The quantum requirement at low irradiance was: 40 mol photons per mol of CO2.4 Bulk stomatal conductance of the ecosystem was maximal in the early morning (0.4‐1.0 mol m−2s−1) and declined over the course of the day as leaf‐to‐air vapour