The intracellular concentration of cAMP in the green alga Chlorella fusca was in the range of 2.10(-9) to 10(-8) moles/g dry weight and was strongly dependent on the growth conditions. The cAMP level was high with high light intensity, low nitrate or glucose concentration. Intracellular cAMP increased only by factor of 2 when high amounts (up to 10(-3) M) of cAMP were added to the medium. Most of the given cAMP was converted to 5'-AMP. Addition of cAMP had little effect on the chlorophyll content of the cells, only at 10(-6) M some enhancement in photoautotrophic cultures was observed. On the other hand high amounts of cAMP in the medium increased the growth rate. DBcAMP* showed a positive effect on chlorophyll synthesis and growth rate at much lower concentrations compared to cAMP. Stimulation effects of exogenous cAMP on the synthesis of chlorophyll were also observed in mixotrophic cultures with a high glucose/nitrate ratio, conditions where chlorophyll synthesis is repressed. Similar to autotrophic conditions DBcAMP was more effective than cAMP. These data indicate that cAMP may act in a system controlling the chlorophyll content of the cells in response to nutrients or light.