The chloroplast development in maize (Zea mays L.) is retarded at low growth temperatures. The composition of thylakoid membranes isolated from fiiliy-expanded leaves of plants grown at 17/14 or 14/12 “C (day/night) differed markedly from that of 25/22 "C-grown plants dne to a deficiency of chloroplast-encoded gene products. Appreciable net synthesis of thylakoid proteins in vivo was observed in a 3-h labelling period with p^SJ-methionine at 14/12 “C, but the ratio of accumuiated chloroplast- to nuclear-encoded products was lesser than at 25/22 °C. The protein synthesis by chloroplasts in vitro demonstrated that the accumulation of thylakoid membrane proteins was markedly temperature-dependent. Both the protein assembly into thylakoids and its subsequent breakdown took plače more rapidly at higher temperatures. Extensive differences in nuclear protein composition were observed between maize leaves grown at 14/12 and 25/22 "C, suggesting a possible role for nuclear factors in suppressing the expression of genes for chloroplast proteins at temperatures which inhibit thylakoid assembly.