The possibility that growth hormone (GH) has effects on long bone growth independent of insulin-like growth factor-I (IGF-I) has long been debated. If this is true, then long bone growth should be more profoundly affected by the absence of GH (since both GH and GH-stimulated IGF-I effects are absent) than by the absence of IGF-I alone (since GH is still present and actually elevated). To test this hypothesis, we compared long bone growth in mice with targeted deletions of Igf1 vs growth hormone receptor (Ghr). Tibial linear growth rate was reduced by approximately 35% in Igf1 null mice and by about 65% in Ghr null mice between postnatal days 20 and 40, a time of peak GH effect during normal longitudinal growth. The Igf1 null mouse growth plate demonstrated significant enlargement of the germinal zone; chondrocyte proliferation and numbers were normal but chondrocyte hypertrophy was significantly reduced. In contrast, the Ghr null mouse germinal zone was hypoplastic, chondrocyte proliferation and numbers were significantly reduced, and chondrocyte hypertrophy was also reduced. We have previously demonstrated that IGF-II is highly expressed in growth plate germinal and proliferative zones, so we considered the possibility that GH-stimulated IGF-II production might promote germinal zone expansion and maintain normal proliferation in the Igf1 null mouse growth plate. Supporting this view, IGF-II mRNA was increased in the Igf1 null mouse and decreased in the Ghr null mouse growth plate.Thus, in the complete absence of IGF-I but in the presence of elevated GH in the Igf1 null mouse, reduction in chondrocyte hypertrophy appears to be the major defect in longitudinal bone growth. In the complete absence of a GH effect in the Ghr null mouse, however, both chondrocyte generation and hypertrophy are compromised, leading to a compound deficit in long bone growth. These observations support dual roles for GH in promoting longitudinal bone growth: an IGF-I-independent role in growth plate chondrocyte generation and an IGF-I-dependent role in promoting chondrocyte hypertrophy. The question of whether GH has direct effects on chondrocyte generation is still not settled, however, since it now appears that IGF-II may medicate some of these effects on the growth plate.