SUMMARY Dunaliella tertiolecta required carbon dioxide in substrate concentrations (1.75 %, v/v) to assimilate either nitrate or nitrite at maximum rates in light. The addition of glucose, glycerol, acetate, pyruvate or a-ketoglutarate did not remove the requirement for carbon dioxide. The rates of nitrate and nitrite assimilation in light depended upon the buffer system used. The lowest rates of nitrate assimilation, I -3 ,umoles/hr/mg. chlorophyll, were observed in 0.05 M-phosphate buffer (pH 7.6) and the highest, 13.7 ,umoles/hr/mg. chlorophyll, in 0.05 M-Tricine buffer (PH 7.6). Nitrite assirnilation was lowest, 7-5 ,umoles/hr/mg. chlorophyll, in 0.05 M-phosphate (PH 7.6) while the highest rates, I 8.7 ,umoles/hr/mg. chlorophyll, were observed in 0.05 M-Tricine (PH 8.6). The low rate of assimilation of nitrate and nitrite in 0.05 M-phosphate buffer (pH 7.6) was increased by diluting the buffer to 0.005 M, at this con- centration the rate in phosphate buffer was comparable to that in tris or Tricine buffers at the same pH values. Buffer type had little effect on either nitrate or nitrite assimilation in the dark. There was no evidence with any buffer system used for the evolution of extra oxygen associated with nitrate or nitrite assimilation in the light. These results provide further evidence for the existence of two independent systems of nitrate reduction, one within and the other without the chloroplast. In addition, they indicate that in Dunaliella tertiolecta the enzyme system which fixes carbon dioxide is unlikely to be the rate-limiting step in photosynthesis.