The coenzyme affinity of lactate dehydrogenase of various parts of rat brain is different to deamino-NAD and NAD as well as to their reduced forms. In direct reactions NAD exhibits a higher activity than deamino-NAD. In the reverse reaction an opposite pattern is observed. The effect of deamino-NADH is much higher than that of NADH. Our studies have shown that the isoenzymes of LDH which are richer in H subunits have a higher affinity for deamino-NAD and deamino-NADH than for NAD and NADH. The isoenzymes of LDH that contain more M forms have opposite properties. LDH-3 does not show a pronounced selective affinity. The data obtained indicate that the activity of LDH and of its 5 isoenzymes varies greatly in different brain parts; crucial changes being observed in the relative percentage of molecular forms of LDH.