The aim of the study was to determine penetration properties of Famotidine fro the formulations through colon adenocarcinoma (Caco)-2 cell monolayers and to compare in vitro with in vivo test results. It also aimed to determine the effect of particle size on the penetration properties of Famotidine when microsphere formulations were used. Famotidine was chosen as a model drug and Caco-2 cell culture model was used. Biodegradable Famotidine microspheres of poly(lactide-co-glycolide)(PLGA) polymer (50:50) were prepared by using multiple emulsion technique. Microspheres were coded according to their particle size and polymer[LHIV: 60 mum Famotidine-PLGA(high viscosity), SHIV: 6 mum Famotidine PLGA(high viscosity), LLIV: 60 mum Famotidine-PLGA (low viscosity), SLIV: 6 mum Famotidine-PLGA (low viscosity)]. Famotidine solution(5 mg/ml) and microsphere formulations were administered orally to mice and blood drug levels were determined and compared with the Caco-2 cell experiments. Permeability values of Famotidine through Caco-2 cells from various formulations were determined (log k(solution)=7,274+/-0,010, log k(SHIV)=-3,884+/-0,033, log k(LHIV)=-2,300+/-0,009, log k(SLIV)=-4,076+/-0,208, log k(LLIV)=3,525+/-0,045). Our results showed that H-2 receptor antagonists alter the barrier properties of the Caco-2 cell monolayer by causing an increment in the tightness of the tight junctions. Therefore, amount of the H-2 receptor antagonist-like drug at the site of action was found to be important as well as polymer type and particle size of microspheres for drug permeation. Permeation of the drug was lower when higher amounts of Famotidine were present at the diffusion site. A controlled release dosage form of H-2 receptor antagonist-like drugs may be beneficial for long-term treatments.