1. The oxidizability of NADPH₂ by quinones in the presence of NADPH₂-diaphorase was tested under aerobic conditions. Also the ^(14)CO₂-fixation rates were compared when Chlorella suspensions were pretreated with 3·10^(-5) M concentration of various quinones for 10 minutes prior and during the ^(14)CO₂-fixation period. 2. A close correlation seems to exist between the rate of NADPH₂ oxidation by quinones and the ^(14)CO₂-fixation rate. The effect of quinones on NADPH₂ oxidation and ^(14)CO₂-fixation were in the order of Dichlone$gt;06-K$gt;NQ$gt;BQ. 3. It is postulated that the phytotoxicity of quinones on Chlorella is due to the deprival of NADPH₂ consequently inhibiting ^(14)CO₂-fixation, thus causing death of the cells. 4. The effect of quinones on amino acids biosynthesis in Chlorella was one of depressed rates, which was especially noted in the case of dichlone. This would be expected from a consideration of NADPH₂ deprival and inhibition of ^(14)CO₂-fixation. Sucrose synthesis was either not affected or rather stimulated, the reasons of which are not clear at the present time.