The relationship between ovarian steroids and LH during the midcycle gonadotropin surge is controversial. Recent demonstration of temporal and quantitative differences in immunoassayable LH (I-LH) and bioassayable LH (B-LH) at midcycle have further clouded this issue. To evaluate the relationship of I-LH, FSH, and ovarian steroids to the onset of the midcycle B-LH surge, blood samples were obtained from five chronically catheterized rhesus monkeys at 2-h intervals for 5-6 days. The plasma was assayed for FSH, LH, 17 beta-estradiol (E2), progesterone (P4), and 17 alpha-hydroxyprogesterone (17-OHP) by RIA and for LH by a rat interstitial cell testosterone in vitro bioassay. The initiation of the B-LH surge served as time zero (to) for the temporal analysis of changes in plasma hormone levels. The I-LH and FSH surges were initiated 6.4 +/- 2.2 h (mean +/- SEM) and 5.2 +/- 1.9 h, respectively, after the onset of the B-LH surge. Although the duration of the ascending limb of the surge was similar for B-LH, I-LH, and FSH, the mean +/- SEM total duration of the B-LH surge (34.5 +/- 3.5 h) was significantly longer (P less than 0.025) than those of I-LH (24.4 +/- 5.0 h) and FSH (27.6 +/- 2.3 h). Before the onset of the B-LH surge (to - 4 h), the ratio of B-H to I-LH was unity; however, during the acme of gonadotropin secretion (to + 12-16 h), the B-LH to I-LH ratio approached 6:1. Doubling times for B-LH, I-LH, and FSH were similar during the ascending phase of the surge. Plasma E2 concentrations increased continuously from to - 40 h to to, with a mean +/- SEM doubling time of 32.6 +/- 4.7 h. Peak E2 concentrations occurred within 6 h after the onset of the B-LH surge. Plasma P4 concentrations began to increase at to - 6 h in four monkeys and at to in one monkey. Plasma P4 concentration plateaued from to to to + 24 h, then increased rapidly, with a mean +/- SEM doubling time of 20.5 +/- 2.9 h. Although there were significant individual variations in plasma 17-OHP concentrations, a definite increase in 17-OHP occurred by to - 10 h, and peak concentrations occurred at to + 1 h.(ABSTRACT TRUNCATED AT 400 WORDS)