Seedlings of alfalfa (Medicago saliva L.) cv. Ranger and cv. Hairy Peruvian were grown under conditions of long day and high temperatures or short day and low temperatures. Under long days and high temperatures, both varieties developed an elongated shoot and exhibited relatively low cold-resistance, as measured by the extent of leakage of cellular substances, after being exposed to sub-zero temperature. Under short days and low temperatures, Ranger seedlings developed a rosette growth and exhibited an improved response to sub-zero temperature. In contrast, Hairy Peruvian seedlings developed an elongated shoot and did not acquire cold-resistance. Hairy Peruvian grown in a nutrient solution containing abscisic acid, however, developed a rosette and acquired an improved response to sub-zero temperature, like cv. Ranger under short days and low temperatures. Addition of gibberellic acid to Ranger seedlings grown under these conditions nullified the environmental effects, since they developed an elongated shoot and did not acquire cold-resistance. It is concluded that abscisic acid and gibberellic acid are involved in the interrelationship between morphogenesis and cold-resistance in the seedlings of the two alfalfa cultivars.