Abstract In many mammalian species, circulating levels of insulinlike growth factor-I (IGF-I) rise during puberty. Previous studies manipulating testosterone levels in rats with normal GH secretion suggested that the pubertal IGF-I rise is regulated by an interaction between GH and sex steroids. Therefore, in a reciprocal study, IGF-I levels were examined during sexual maturation of the GH-deficient dwarf (dw/dw) rat which has a selective genetic deficiency of GH but normal sex steroid levels. Male dw/dw rats were treated with daily injections of recombinant human GH (200 μg/100 g body weight) or saline vehicle, from 28 to 70 days of age. Sexual maturation was determined to occur primarily between 42 and 63 days of age based on testis and seminal vesicle growth and plasma testosterone levels. GH treatment had no effect on seminal vesicle weights, plasma testosterone or gonadotrophins. GH administration resulted in a 7% increase in absolute testes weight (P<0·05), but a 50% increase in body weight (P<0·0001). These results supported previous findings that the reproductive development of dw/dw rats is essentially normal. Untreated dw/dw rats had no rise in IGF-I levels during sexual maturation. In contrast, treatment with GH produced a marked sustained rise in IGF-I levels (P<0·0001). Ligand blots demonstrated GH induction of IGF-binding protein-3 (IGFBP-3) and an IGFBP cluster at 32 kDa. The initially high immunoreactive IGFBP-1 levels (>600 ng/ml) decreased by 49 days of age after which untreated dw/dw rats had significantly higher IGFBP-1 levels than GH-treated dw/dw rats (P<0·01). We conclude that GH secretion, rather than sex steroids, may be the predominant determinant of pubertal IGF-I levels in rats and that the rise in circulating IGF-I levels during puberty is not an indispensible event for normal reproductive development. Journal of Endocrinology (1994) 141, 393–401