To evaluate oral mucosal absorption of drugs in dogs, a newly designed in situ perfusion system with a circulating perfusion chamber was developed. The utility of the perfusion system was investigated by using three drugs: salicylic acid (SA), sulfadimethoxine (SM), and diltiazem (DIL). The oral mucosal absorption of the drugs could be adequately described by first-order rate processes. The absorption rate was independent of the amount of un-ionized drug, which varied with the pH of the solution. The absorption of SA was similar for various oral mucosal sites and for repeated experiments using the same site. Pharmacokinetic analysis for the plasma or medium concentration of SA after perfusion showed that SA was absorbed at the rate constant of 0.071 h-1, and that approximately 70% of SA absorbed from oral mucosa was transferred to the circulating blood.