Young seedlings of Zea mays L. scarcely grow at temperatures of 15 °C or lower. The leaves are yellowish, the chlorophyll concentration being lower than 10 ug (a+b) per cm2, and the rate of photosynthesis is negligible. When a period of low temperature is given to older plants, which already have some green leaf surface, the reduction in relative growth rate and photosynthesis is less severe. Only those leaf parts that elongate during the cold treatment are yellow. It can be shown that the day temperature is the important factor in influencing the chlorophyll concentration in growing leaf parts. Plants subjected to low night temperatures remain fully green and their growth rate is virtually unaffected. The data suggest that photosynthesis rate is reduced at concentrations of chlorophyll (a+b) below 40p.g per cm'2. When plants are brought back to temperatures of 20° or higher both photosynthesis rate and growth rate soon reach values normal for this temperature.