Blood samples were collected prior to vaginal hysterectomy from nine women in the proliferative phase of the cycle and from 15 women in the secretory phase, and from each uterus five grams of the fundus were obtained. The concentrations of progesterone (P), 20alpha-dihydroprogesterone (20alpha-DHP), estradiol-17beta (E2), and estriol (E3) were determined in serum and in the tissue with the use of specific radioimmunoassays. The concentrations of P and 20alpha-DHP in serum and myometrium were significantly higher in the secretory than in the proliferative phase. Furthermore, the concentration of P in myometrium was significantly higher than in serum in the secretory phase, indicating a specific uptake of P by the myometrium. The concentration of E2 was about 10 times higher in the myometrium than in the serum during both phases of the cycle and appeared to be independent of the serum levels. This indicates a specific but limited uptake of E2 by the myometrium. The E3 levels in the tissue showed no significant differences during the menstrual cycle, whereas in the serum the E3 concentrations were higher in the secretory than in the proliferative phase of the cycle.