Thalli of Ramalina maciformis were moistened to their maximal water holding capacity, thus, simulating actual conditions following a heavy rainfall. Time courses of net photosynthesis at 17° C and 750 μE m-2 s-1 light intensity (PAR) were obtained during drying of the thalli. At ambient CO2 concentrations from 200 to 1,000 ppm, CO2 uptake of the moist lichens was depressed at high water content. After a certain water loss, net photosynthesis increased to a maximal value and decreased again with further drying of the thalli. The degree of initial depression of photosynthesis decreased with increasing ambient CO2 concentration, and it was fully absent at 1,600 ppm ambient CO2. Under these conditions of CO2 saturation, net photosynthesis remained constant at maximum for many hours and decreased only when substantial amounts of water had been lost. We conclude that the carboxylation capacity of the lichen is not affected by high contents of liquid water. Therefore, the depression of CO2 uptake of the water saturated lichen at lower (e.g. natural) ambient CO2 must be due exclusively to increased resistance to CO2 diffusion from the external air to the sites of carboxylation.