Abstract. An automatic closed chamber system for measuring net carbon flux from the forest floor was equipped with both a transparent and an opaque cover. The system was operated in such way that a measurement session with transparent chamber was followed by a session with dark chamber. This made it possible to estimate besides total daytime respiration and nighttime respiration also the gross assimilation of the vegetation enclosed in the chamber. The chamber was used at two locations, Hyytiäla in Finland and Norunda in Sweden. Results were compared to estimation of gross assimilation by extrapolation of nighttime respiration and the difference between daytime and nighttime respiration was analyzed. Estimated gross photosynthesis from the darkening sessions by the chamber resulted in a higher gross photosynthesis then obtained by extrapolation from nighttime respiration for Norunda, but not for Hyytälä. Comparison of obtained gross photosynthesis rates indicated that the forest floor vegetation contributed up to 30% of maximum net ecosystem uptake.