The regulation of the synthesis of one of the major polypeptides of chloroplast membranes in Chlamydomonas reinhardi y-1 has been studied in order to determine what factors are involved in the control mechanism. The polypeptide is synthesized in the cytoplasm and previously was designated as c (J. K Hoober. 1972. J. Cell Biol.52:84). Under normal conditions the synthesis of polypeptide c appears to be coupled to the synthesis of chlorophyll. When greening cells are illuminated through a light filter opaque below 675 mmicro, the conversion of protochlorophyllide to chlorophyllide is blocked. Although this elimination of light below 675 mmicro, does not affect, in the main, protein synthesis in the chloroplast and cytoplasm, synthesis of polypeptide c is inhibited. Also, control cells synthesize neither chlorophyll nor polypeptide c in the dark. However, when cells are treated with chloramphenicol, an inhibitor of chloroplast protein synthesis, the synthesis of polypeptide c occurs in the absence of light required for chlorophyll synthesis. Chlorophyll per se does not appear to be required for synthesis of polypeptide c, since treating cells with hemin, maleate, or malonate causes an inhibition of the synthesis of chlorophyll but not of polypeptide c. The results of these experiments are discussed in terms of a proposed mechanism by which synthesis of polypeptide c is regulated at the transcriptional level by a precursor of chlorophyll, and this regulation is mediated by a protein or proteins synthesized within the chloroplast.