Chlamydomonas reinhardi did not assimilate nitrate or ammonium unless a suitable source of carbon was provided. Suitable sources were CO2 in light or, in darkness, acetate or the carbon reserves accumulated in nitrogen-starved cells. Illuminated cells in which CO2, assimilation was prevented by DCMU did not assimilate either nitrate or ammonium but did so when acetate was added; the rate of nitrogen assimilation was faster than with acetate in darkness and it is suggested that light promoted nitrogen assimilation by photophosphorylation. The assimilation of nitrate was inhibited by addition of either ammonium or nitrite and that of nitrite by addition of ammonium. Experimental evidence indicated that products of ammonium assimilation, rather than ammonium itself, were inhibiting.