Endothelin has various paracrine and endocrine effects on the male reproductive system. Testosterone is probably responsible for the higher endothelin levels in males. In addition, there is much ambiguity about the relationship between gonadotrophic hormones and endothelin. In order to study in more detail the relationship of endothelin with the hypothalamo-pituitary-gonadal axis in the male, we investigated 18 male patients with various forms of hypogonadism (seven with hypergonadotrophic hypogonadism and 11 with hypogonadotrophic hypogonadism). Eight age-matched healthy males served as controls. The basal endothelin levels in patients with hypogonadism (0.95 +/- 0.53 fmol ml(-1)) were significantly higher than those of the controls (0.54 +/- 0.06 fmol ml(-1); P < 0.05). Males with hypergonadotrophic hypogonadism had significantly increased endothelin concentrations (1.05 +/- 0.57 fmol ml(-1); P < 0.05), whereas those with hypogonadotrophic hypogonadism (0.89 +/- 0.53 fmol ml(-1)) had nonsignificantly (P > 0.05) elevated levels. No significant correlation was found between plasma endothelin levels and gonadotrophin, prolactin and testosterone concentrations. The results of this study suggest that plasma endothelin levels are increased in males with hypogonadism.