Attempts were made to see if feeding of ginseng ethanol extract could affect proper- ties of rat brain lactate dehydrogenase such as specific activity, heat stability, Km for substrate, inactivation by 3-bromopyruvate and trypsin, and immune response. The following results were obtained. Specific activity of LDH was observed to reach maximum in 5 month after birth and then decrease steadily. However, that of LDH from rat fed with ginseng ethanol extract was found in rat fed with ginseng ethanol extract. 3-bromopyruvate was shown to inactivate LDH-5 from old rat fed. Inactivation of LDH-5 by trypsin was remarkable in old rat fed. Km value for pyruvate in old rat fed was remarkably decreased. Cumulative results suggest that ginseng ethanol extract could affect conformational change of LDH responsible for altered properties through unknown mechanism.