Animal experiments and clinical studies on the interactions between growth hormone (GH) and the male hypothalamic-pituitary-gonadal axis have predominantly concentrated on GH and sex steroid interactions in the regulation of growth and development, or on the metabolic effects of GH. In contrast, little attention has been paid to the possible effects of GH on spermatogenesis, although the first report dealing with this topic was published almost 30 years ago. The interactions of GH and its main mediator, insulin-like growth factor I (IGF-I), with the hypothalamic-pituitary-gonadal axis, and their role in spermatogenesis have recently been investigated using in vitro systems and different animal models (mice and rats). Using Leydig and Sertoli cell cultures, complex interactions between GH/IGF-I and the gonadotropins affecting differentiated cell functions, e.g. steroidogenesis and cell division, have been demonstrated at the cellular level. In vivo studies using immature and mature hypophysectomized rats and GH-deficient mutant male mice and rats indicate that IGF-I can play an important role in the regulation of steroidogenesis and spermatogenesis. Furthermore, although follicle-stimulating hormone and luteinizing hormone are the major regulators of testicular IGF-I production, GH may play an indirect role by potentiating the actions of the gonadotropins in regulating testicular IGF-I content. A large proportion of men presenting at male-infertility clinics are diagnosed as having idiopathic infertility. Further studies are necessary to investigate whether defects associated with GH and/or IGF-I effects in the testis are the cause of male infertility in a small group of these patients.