Abstract: The in vivo effects of gliclazide and metformin HCl on plasma concentration of caffeine have been studied in rats. The plasma concentration of caffeine was determined by UV spectrophotometry after oral single administration of caffeine alone and with gliclazide and metformin HCl. The in vivo study for determination of plasma concentration of caffeine showed that concurrent administration of caffeine and gliclazide have not made noticeable changes in plasma concentration of caffeine. But administration of caffeine and metformin HCl has showed a significant change in plasma concentration of caffeine. So, a competitive inhibition of the binding to plasma protein by metformin HCI increases the plasma concentration of caffeine. Thus any change in plasma concentration may affect the pharmacological or toxic effects of the drug.