The effect of a phenacyl imidazolium compound (LY177507 or Proglycosyn, Eli Lilly) on the direct (glucose-->glucose-6-phosphate-->glycogen) and indirect (three-carbon compounds-->glucose-6-phosphate-->glycogen) pathways of liver glycogen synthesis was studied in conscious rats. [1-13C]Glucose (99% enriched) was infused intraduodenally into chronically catheterized Proglycosyn-treated (n = 7) and saline-treated (n = 7) rats for 120 min. Net hepatic glycogen synthetic rates were increased twofold in drug-treated rats compared with saline-treated controls. The percentage of liver glycogen synthesized by the direct pathway was calculated by comparing the 13C isotopic enrichment in the C1 and C6 positions of hepatic glycogen and plasma glucose using 13C nuclear magnetic resonance spectroscopy and gas chromatography-mass spectroscopy techniques and was found to be 59 +/- 5% and 39 +/- 2% (P < 0.05) in the saline treated and Proglycosyn-treated groups, respectively. Net flux rates for the direct and indirect pathways were calculated to be 0.24 +/- 0.04 and 0.17 +/- 0.03 mumol/g liver per min, respectively, in the saline-treated group and 0.30 +/- 0.04 (P = NS) and 0.46 +/- 0.06 (P < 0.05) mumol/g liver per min, respectively, in the Proglycosyn-treated group. Thus, Proglycosyn increases net hepatic glycogen synthesis in vivo exclusively through augmentation of the indirect pathway.