The effects of a subminimal temperature on germinating Zea mays kernels have been studied at the physiological and ultrastructural levels. After 72 h of germination at 16 °C, kernels were exposed to 4 °C for intervals ranging from 4 to 25 days and then returned to 16 °C. Resumption of growth of all the embryos occurred when the cold exposure was 6 or 8 days. When periods at 4 °C were longer than 8 days, a decrease in the percentage of embryos able to grow was observed; this was proportional to the length of cold exposure. After 26 days or more at 4 °C, all the embryos died. An ultrastructural study of the primary root cells showed that modifications occur in the cytoplasm and in the nucleolus. The nucleolus became predominantly fibrillar and unusual ribonucleoproteinic granules were frequently observed in and near the nucleolus organizer region. These changes were related to a strong decrease in transcription.