The total LDH activity in the uterine fluid of mice shows a rising trend from the first day after mating (DAM-1) to the seventh day after mating (DAM-7). This suggests that LDH activity increases as gestation progresses. During early pregnancy, M-isozymes of LDH show a predominance from DAM-1 to DAM-3 in the uterine luminal fluid of mice, while H-isozyme shows a rising level from DAM-5 (implantation day) and attains a maximum level at DAM-7 (the post-implantation period). Such shift of M-isozymes into H-isozymes of LDH (lactate dehydrogenase) during pre-implantation (DAM-3) to post-implantation (DAM-7) period changes the uterine luminal environment from anaerobic to aerobic condition which is more conducive for the normal development, implantation and survival of the growing blastocyst.