We explored to determine if iNOS could be induced by insulin in osteoblast-like UMR-106 cells. Insulin (100 nM) stimulated nitric oxide production by twofold and significantly increased iNOS mRNA and protein levels. Insulin also increased collagen synthesis, but had little effect on alkaline phosphatase activity. In contrast, IGF-1 had little effect on NO production below 10 nM and it stimulated NO production by only 57% at 100 nM. IGF-1 had little effect on collagen levels, whereas it inhibited alkaline phosphatase activities in a dose-dependent manner. When an MEK inhibitor was preincubated, insulin failed to stimulate NO production, whereas insulin dramatically increased NO production in the ERK1 overexpressed cells. Taken together, it is proposed that insulin increases iNOS mRNA, iNOS protein, and NO production, possibly via activation of ERK. These may play an important role in osteoblast functions such as collagen synthesis.