Nitric oxide (NO) is a short-lived free radical that plays an important regulatory role in several biological processes. Cytokines such as interleukin-1, tumor necrosis factor, and interferon-gamma have been shown to stimulate NO production in many cells types. Although these cytokines are known to have potent effects on bone remodeling and osteoblast function, the role of NO as an effector molecule in bone has been little studied. Here we investigate the effects of cytokines and calciotropic hormones on NO production by human osteoblast-like cells (hOB) and the role of NO as a modulator of osteoblast growth. Unstimulated hOB produced little NO, as reflected by measurement of nitrite concentrations in hOB-conditioned medium. NO production was not significantly altered by PTH and 1,25-dihydroxyvitamin D or human recombinant interleukin-1 beta (10 U/ml), tumor necrosis factor-alpha (25 ng/ml), and interferon-gamma (100 U/ml) individually. Combinations of all three cytokines at these concentrations, however, dramatically increased both NO generation and cGMP production. The stimulatory effect of cytokines on NO production began 12 h after exposure and was inhibited by cycloheximide, actinomycin-D, dexamethasone, and the competitive inhibitor of NO synthase L-NG-monomethylarginine. Reverse transcription/polymerase chain reaction analysis of hOB RNA, followed by direct sequencing of the amplified products, showed that hOB express the inducible, rather than the endothelial or neuronal, forms of NO synthase. Cytokine-induced increases in NO production were associated with a marked inhibition of [3H]thymidine uptake to less than 10% of that observed in control cultures. Abrogation of NO synthesis with L-NG-monomethylarginine under these conditions significantly increased [3H]thymidine uptake to approximately 20% of the control value, suggesting that NO may partly be responsible for the inhibition of osteoblast proliferation induced by these cytokines. Our data indicate that proinflammatory cytokines induce NO production in osteoblast-like cells and show and that this mediator plays a role in regulating cell growth. These findings may have important implications for the pathogenesis and management of bone loss in diseases associated with cytokine activation, such as rheumatoid arthritis.