Indomethacin, an inhibitor of prostaglandin synthesis that modulates cytokine production, increases hepatic glucose output (HGO) in humans. However, prostaglandins stimulate glucose production in vitro. To investigate the mechanism of HGO stimulation by indomethacin, we compared the effect of pentoxifylline, an inhibitor of cytokine production, versus saline (study 1, n = 6) and of indomethacin versus the combination of indomethacin and pentoxifylline (study 2, n = 5) on basal HGO. HGO was measured by primed, continuous infusion of 3-3H-glucose. In study 1, pentoxifylline infusion resulted in an immediate, transient decrease of HGO of approximately 50% (from 12.9 +/- 0.4 to 6.0 +/- 1.7 micromol/kg/min after 15 minutes, P < .03 v control). There were no differences in concentrations of glucoregulatory hormones between the two experiments. In study 2, after indomethacin administration, HGO increased transiently by approximately 84% (from 9.7 +/- 0.7 at baseline to 16.7 +/- 2.4 micromol/kg/min after 135 minutes, P < .05). However, pentoxifylline did not affect the increase in HGO induced by indomethacin. There were no differences in concentrations of glucoregulatory hormones between the two experiments. Therefore, indomethacin stimulates HGO by mechanisms unrelated to glucoregulatory hormones, prostaglandins, or cytokines.