The effects of fasting on the intestinal absorption profiles of salicylic acid, barbital, haloperidol, and chlorpromazine were studied in anesthetized rats. The in situ technique employed in this study yielded absorption rate constants which were realistic and comparable to those observed following oral drug administration. The weak acids, barbital and salicylic acid, were found to obey first-order kinetics throughout the experiments. The highly lipidsoluble weak bases, haloperidol and chlorpromazine, obeyed firstorder kinetics after the first 10 min. of experimentation. No apparent deviation in absorption patterns occurred when fasting periods were less than 20 hr. However, when the period of inanition exceeded 20 hr., absorption rates were found to decrease significantly and the decrease was dependent on the duration of the fasting period. It is possible that the unusual drug absorption patterns noted in these studies could be accounted for by one or more of the various physiological and/or biochemical changes which occur within an organism subjected to conditions of prolonged fasting.