The effects of LHRH stimulation upon plasma LH levels measured by bioassay and radioimmunoassay were examined during the normal menstrual cycle, and in normal men and post-menopausal women. After administration of 100 mug LHRH by subcutaneous injection during the early follicular phase, a 2.7-fold rise in bioactive serum LH to 58 +/- 18 mIU/ml at 30-60 min was accompanied by an equivalent rise in immunoreactive LH, with unchanged bio:immuno (B:I) ratio of 0.9 +/ %/- 0.2 (SD). During the late follicular phase, bioactive serum LH rose 8-fold to 258 +/- 120 mIU/ml at 30-180 min, and the B:I ratio was significantly increased from 1.7 to 2.5. During the luteal phase, bioactive LH values rose 8.1-fold to 223 +/- 97 mIU/ml at 30-60 min, with increase in B:I ratio from 1.1 to 1.8. The LHRH-stimulated serum LH levels declined more rapidly in the early follicular phase than during the late follicular and luteal phases. Elevations of circulating LH concentrations following LHRH administration during the normal menstrual cycle were usually accompanied by a significant rise in B:I ratio, except during the early follic ular phase when the LH responses were small and the B:I ratio did not change. After LHRH stimulation of serum LH levels in men and post-menopausal women, relatively small and inconstant elevations of B:I ratio were observed above the basal value of 2.9. Although the B:I ratio was usually close to unity in cycling women, the ratio was from 2 to 3 in men and postmenopausal women, and during LHRH stimulation of normal women at the late follicular and luteal phases of the menstrual cycle. Thus, the increased LH secretion rate of post-menopausal women and LHRH-stimulated cycling women was frequently accompanied by a rise in the B:I ratio. These observations suggest that circulating LH molecules exhibit a relatively higher biological activity during states of increased biosynthesis and release of gonadotropins.