AbstractThere are two types (M and H) of lactic dehydrogenase that are found in most animals. The M‐type is found largely in the more anaerobic tissues, such as voluntary skeletal muscles; the H‐type is found in aerobic tissues, such as cardiac muscle. In the immature uterus, the level of M units is considerably less than that found in the mature uterus. Injection of estradiol leads to a marked increase in M units in the immature uterus, but there is no significant change in the concentration of the H form. Testosterone and progesterone, in contrast, promote a proportional increase in the two lactic dehydrogenases in the immature uterus. Testosterone, however, induces a selective synthesis of M units in the seminal vessels of the immature rat. Hypophysectomy leads to a decrease in M units of skeletal muscle. The effects of various hormones on the composition of lactic dehydrogenase of the rat and chicken will be summari