SummaryThe LDH activities of serum and homogenated rat heart, kidney and liver during development were measured by the method of Wrbblewski and the isozyme patterns observed by the agar gel electrophoresis and the following results were obtained.1) LDH activityIn rat serum, no significant differences were indicated among the activities in six age groups during development.The measurement of the activity in these homogenated tissues revealed the highest in heart and the fairly high titers in kidney and liver. In all tissues, LDH activity changed during development showing the relatively low titer in heart and kidney and the higher in liver in G4.2) LDH isozymeThe LDH isozyme patterns of each tissues gradually changed during development by different course observed in the human tissues.LDH‐5 was the most principal isozyme in rat serum and LDH‐4 and LDH‐5 were consistently dominant through the whole life.In homogenated heart, LDH‐3 was dominant in the early life period and the LDH isozyme patterns changed gradually to H type predominance.In kidney, LDH‐5 was the major isozyme in the early life period and isozyme patterns gradually alterated to H type predominance showing relatively dominant in LDH‐5 ares.In rat liver, LDH‐5 was also the major isozyme in the early life period and difinitely kept the predominance to the adult period.3) The mechanisms of these phenomena w