The acute effects after administration of 3-O-Methyl-D-Chiro-inositol, D-Chiro-inositol and manganese, components of the pH 2.0 putative mediator of insulin action, on plasma glucose were examined in low dose streptozotocin-treated rats. 3-O-Methyl-D-Chiro-inositol at a bolus dose of 5 mg/kg decreased plasma glucose 6% at 120 min (p < 0.05). A higher bolus dose of 3-O-Methyl-D-Chiro-inositol (15 mg/kg) promoted a more persistent hypoglycemic effect of 22% at 120 min (p < 0.05). Infusion of 8.3 microg/min of manganese chloride lowered plasma glucose by 23% (p < 0.05). 3-O-Methyl-D-Chiro-inositol (15 mg/kg) together with manganese chloride (8.3 microg/min) promoted a reduction of 49% in 120 min (p < 0.05). D-Chiro-inositol at a bolus dose of 5 mg/kg had no effect. A single dose of 15 mg/kg produced a reduction of 21% (p < 0.05) in 120 min. D-Chiro-inositol (15 mg/kg) associated with manganese chloride (8.3 microg/min) decreased elevated plasma glucose 47% (p < 0.05) in 120 min. D-Chiro-inositol coadministered with manganese reduced glucose concentrations during the final 60 min (p < 0.05). 3-O-Methyl-D-Chiro-inositol and D-chiro-inositol are components of the mediator structure. Manganese is also a presumed component of the mediator, having an important role in glucose uptake, insulin release and mediator generation. These compounds have also been identified in the literature as hypoglycemic agents.