Assimilation and excretion of ammonium by green and algae-free Paramecium bursaria as well as by isolatedly grown endosymbiotic Chlorella spec, were determined as also the specific activities of GOGAT and Glutaminase. The glutamine uptake was followed after incubation with 14C-glutamine. Ammonium is assimilated by green Paramecium bursaria as well as by isolated Chlorella spec. only in the light. This process is inhibited by DCMU. The uptake of ammonium by the chlorellae depends on pH, the possible role of which in the regulation of algal growth within the symbiotic unit is discussed. In addition also a glutamine/glutamate shuttle between the Paramecium and the endosymbiotic algae is present furnishing nitrogen to the endosymbiotic cells. However, the uptake-rate for glutamine is lower than the one for ammonium. This might indicate that glutamine is a less suitable nitrogen source for the endosymbiotic algae within the symbiotic unit.