The concentration of luteinizing hormone-releasing hormone (LH-RH) in both cerebrospinal fluid (CSF) and plasma in 36 women with various conditions were determined, and the correlation of LH-RH concentrations between plasma and CSF was studied. Paired samples of plasma and CSF were obtained just before operation and were extracted with methanol for LH-RH radioimmunoassay. LH and FSH were also measured by radioimmunoassay. The mean plasma LH-RH levels were 3.79 pg/ml in the follicular phase, 4.15 pg/ml in the luteal phase, 12.73 pg/ml in postmenopausal women and 2.31 pg/ml in pregnant women at term. No immunoreactive LH-RH in the CSF was found in 29 out of 36 subjects, and the remaining 7 subjects showed very low levels (less than 0.95 pg/ml). The LH-RH levels in the CSF of post-menopausal women were not higher their those in the remaining 3 groups, although their plasma levels of LH-RH were significantly (p less than 0.001) elevated. The mean plasma levels of LH and FSH in post-menopausal women were significantly higher than those in normal cyclic women and in pregnant women. The present results suggest that little or no LH-RH is present in the CSF, and it is not clear that there is any correlation between the LH-RH level of plasma and those of CSF.