The mechanism in the intestinal absorption of coumarin and 7-hydroxycoumarin were studied using the in vitro everted and non-everted gut of guinea pigs and rats. The results of the experiments indicated that both coumarin and 7-hydroxycoumarin are absorbed by the passive drug-transfer process. The presence of probenecid/KCN increased the amount of drug transferred across the gut. The mechanism of the latter compounds on the drug-transfer process is still not clear.