When the plasma concentrations of estrone sulfate (E1S) were measured in five menstrual cycles, the highest concentrations were found on the day of LH peak (14.25 nmol/l +/- 2.94 [SE]). Peak levels of E1S were 20 times higher than the highest E2 levels measured (0.769 +/- 0.276 nmol/l). To determine whether E1S can be metabolized by adult and fetal tissues we examined estrone (E1) sulfatase activity in brain and other tissues. E1 Sulfatase activity was present in all tissues studied including adult endometrium, fat and skin. When the rate of sulfatase activity was measured in homogenates of fetal hypothalamus, frontal cortex and pituitary (n = 4), the hypothalamic activity (306.0 +/- 39.1 [SE] pmol/min/mg protein) was significantly higher than that of the frontal cortex (127.4 +/- 19.4, P less than 0.002) or pituitary (193.7 +/- 43.3, P less than 0.03). This was not apparent in the adult (n = 2) where the enzyme activity was similar in the hypothalamus (413.9 +/- 27.3) and frontal cortex (446.3 +/- 82.2) and lower in the pituitary (98.2 +/- 19.2). The Km for E1 sulfatase in the fetal frontal cortex was 28.9 microM. The high E1 sulfatase activity in estrogen responsive target tissues, particularly fetal hypothalamus, accompanied by a large circulating reservoir of E1S, suggest that this enzyme could possibly have a regulatory role in controlling the level of intracellular estrogens and in modulating their intracellular function.