Plasma gonadutropin levels throughout the regular menstrual cycle in 10 Japanese women were measured daily using radioimmunoassay. At the peak of ovulation, mean FSII levels were 17.6 ± 7.9 mlU/ml and mean LH levels were 75.2 ± 26.0 mlU/ml. At midcycle, the mean gonadotropin levels were significantly loner in Japanese nomen than in Nigerian women who, as reported by Nylander (1973), had a high frequency of thinning. It is, therefore, suggested that the low frequency of ditygotic thinning in Japanese women might be related to their low output of gonadotropin.