Both growth hormone and IGF-I stimulate islet cell growth, inhibit cell apoptosis and regulate insulin biosynthesis and secretion. Growth hormone receptor gene deficiency (GHR^-/-) caused diminished pancreatic islet cell mass and serum insulin level and elevated insulin sensitivity. As IGF-I gene expression was nearly abolished in these mice, we sought to determine whether that had caused the islet defects. To restore IGF-I level, we have generated transgenic mice that express rat IGF-I cDNA under the direction of rat insulin promoter 1 (RIP-IGF). Using RNase protection assay and immunohistochemistry, the IGF-I transgene expression was revealed specifically in pancreatic islets of the RIP-IGF mice, which exhibited normal growth and development and possess no abnormalities in glucose homeostasis, insulin production and islet cell mass. GHR^-/- mice exhibited 50% reduction in the ratio of islet cell mass to body weight, increased insulin sensitivity but impaired glucose tolerance. Compared with GHR^-/- alone, IGF-I overexpression on GHR^-/- background caused no change in the diminished blood glucose and serum insulin levels, pancreatic insulin contents and insulin tolerance, but improved glucose tolerance and insulin secretion. Remarkably, islet-specific overexpression of IGF-I gene in GHR^-/- mice restored islet cell mass, at least partially through cell hypertrophy. Interestingly, male mice of double transgenic demonstrated a transient rescue in growth rates vs. GHR^-/- alone, at 2-3 months of age. Our results suggest that IGF-I deficiency is part of the underlying mechanism of diminished islet growth in GHR^-/- mice and are consistent with the notion that IGF-I mediate growth hormone-induced islet cell growth.