Changes in circulating inhibin levels were related to changes in testosterone (T) and the gonadotrophins luteinizing hormone (LH) and follicle-stimulating hormone (FSH) in a hypogonadotrophic hypogonadal man before and during pulsatile gonadotrophin-releasing hormone therapy which resulted in normal spermatogenesis. Before treatment, the plasma inhibin levels in the patient (210 ± 50 U/l; mean ± SD of four samples) were lower than in normal controls (552 ± 150 U/l; p < 0.01), as were T (1.1 nmol/l) and gonadotrophin ( < 1.0 IU/1) levels. Within 1 week of gonadotrophin-releasing hormone treatment, plasma LH (14.1 ± 0.7IU/1) and FSH (14.4 ± 0.6 IU/1) reached supraphysiological levels. In response, T and inhibin concentrations increased progressively to reach high normal levels (27.7 ± 1.6 nmol/l and 609 ± 140 U/l) at 4 weeks, by which time the gonadotrophin levels started to decline and gradually returned to the normal range between 12 and 24 weeks of treatment. There was a concomitant decrease in T and inhibin levels which remained within the normal range. The decline in the FSH level following the rise in testicular hormones was earlier and steeper than that of LH (37.5 % decrease at 4 weeks vs. 30.4 % at 12 weeks), suggesting that T and inhibin may act together to inhibit pituitary FSH secretion as opposed to LH secretion which is primarily controlled by T. It is concluded that, in man, during maturation of the pituitary-testicular axis, changes in circulating inhibin parallel those of T, and quantitatively normal inhibin secretion is dependent on gonadotrophin stimulation. FSH secretion may be regulated through negative feedback control, by both T and inhibin.