The lipids of alfalfa were analyzed during artificial hardening. An increase in fatty acids, particularly linoleic acid; phospholipids; and triglycerides was observed in leaves, stems, and roots. The largest increase in phospholipids, rich in linoleic acid, occurred in the roots. The desaturation of the fatty acids was increased; this increase in desaturation was slight in the phospholipids, but pronounced in the triglycerides. The galactolipids were apparently not affected by the treatment. The level of trans-3-hexadecenoic acid, which is present only in phosphatidyl glycerol, was markedly depressed during hardening.