Estimations of immunoreactive LHRH [luteinizing hormone releasing hormone] and LH [luteinizing hormone] in pooled sera of girls, adult women and postmenopausal women were carried out. The girls were divided into 3 groups: I - girls aged 2-4 yr, II - girls aged 5-8 yr and III - girls 9-12 yr of age. The estimated concentrations of LHRH in particular groups were as follows: in group I - 1.2 .+-. 0.2 pg/ml, in group II - 2.2 .+-. 0.4 pg/ml, in group III 31.0 .+-. 4.4 pg/ml in adult women 6.3 .+-. 1.8 pg/ml and in postmenopausal women 16.6 .+-. 2.4 pg/ml. The concentrations of LH in the same groups were 4.3 .+-. 0.7; 4.5 .+-. 0.8; 11.0 .+-. 1.4, 23.3 .+-. 2.4; and 120.0 .+-. 14.7 mIU/ml, respectively. The sexual maturation of girls apparently is initiated by the enhanced hypothalamic activity, reflected in higher concentrations of immunoreactive LHRH in peripheral serum.