The plasma levels of proteins bound to steroids come under the influence of the levels of estrogens. This is why we have studied the changes in plasma SHBG in sterile women who were being treated by gonadotropins (HMG-HCG). The plasma level of SHBG was worked out using Laurell's electroimmunoassay technique with an autoradiographic display using estradiol labeled with I.125. It only takes 48 hours contact with radio films. Out of 12 patients 9 responded to the treatment as demonstrated by the plasma levels of estradiol (E2) and progesterone (P). We have found that in these women there was a rise in plasma SHBG which was significant from the 7th day of the treatment onwards (3.7 +/- 1.7 mg/l as against 5.9 +/- 2.6 mg/l, p less than 0.01) and this reached nearly twice the base level when E2 reached its maximum level (6.4 +/- 2.7 mg/l). When treatment was stopped the plasma level of SHBG dropped to that of the follicular phase, but it was out of step with the plasma E2 levels. The index of free E2 as compared with E2/TEBG showed a close superimposition when compared with the levels of E2 obtained using a radioimmunological method. This fact reinforces the value of plasma E2 as the way of monitoring the effect of induction of ovulation. Our overall conclusion is that a marked and rapid change in the level of plasma E2 directly influences the level of plasma SHBG. The consequences for treatment are that it is important to respect as far as possible the physiological conditions that are present when induction of ovulation is undertaken.