The paper tries to examine the population variation in the pattern of menstruation between two linguistically cognate tribes of North East India, namely, the Mishing tribe of Brahmaputra river valley (plains) of Assam and the Minyong tribe of the hilly terrains of Arunachal Pradesh. The common belief for the cause of menarche and menstruation is found to be ‘a process of blood purification’ among the Mishings and ‘a sign of virginity’ among the Minyongs. The Minyong couples belief that at the time of conception the menstrual blood coagulates in the body of the embryo. The difference between the mean ages at menarche between the tribes is not found to be statistically significant. However, the correlation matrix shows statistically significant relationships between ages at menarche, marriage and first delivery among the Minyong mothers but not among the Mishing mothers. The study shows a declining trend in the mean ages of menarche from the mothers of the older age groups to the younger age groups in both the tribes. Within woman variation is observed in the menstrual cycle length before and after delivery of the first child as well as in the duration of menstruation.