The influence of low temperature (3 degrees C.) on development of submicroscopic structure in plastids of Zea m. leaves was studied. Leaves from 8-day old etiolated plants, with plastids showing the prolamellar body and few lamellae, were floated for 1 day on tap water both in the dark and in the light, at 26 degrees C and at 3 degrees C. The structures remain unchanged in the dark, independent of temperature. Whereas in the light at 26 degrees C., normal development of parallel compound lamellae and formation of grana occurs, in light at 3 degrees C. ring structures are formed. Under the latter conditions protochlorophyll is converted to chlorophyll, although the in situ absorption maximum is different from the one for chlorophyll in plants grown in light at 26 degrees C. When leaves were transferred from light at 3 degrees C. to light at 26 degrees C., ring structures in the plastids disappeared and normal development occurred. The possibility is discussed that development of parallel-arranged compound lamellae is due both to photochemical and synthetic processes, involving not only accumulation of chlorophyll, but also synthesis of other compounds.