Five experimental models of drug absorption were evaluated using chloroquine and phenobarbitone as tracer drugs. Their absorption was followed at different pH values and in the absence and presence of the interactants kaolin and activated charcoal. The experimental models tested were: an in vivo buccal partitioning technique, an in situ rat intestine technique, an everted rat intestine method, the Sartorius absorption simulator and a centrifugation method in which adsorption of drug to an interactant was measured. Qualitatively similar results were obtained using all five techniques. The data, however, indicated that the buccal partitioning and the in situ intestinal models were superior to the other techniques. Chloroquine and phenobarbitone appeared to be adsorbed to the Sartorius membrane, the amount of drug absorbed using everted rat intestine was small while the centrifugation method does not involve an absorption phase.