A study has been made of the transport of seven morphine analogs across short-circuited preparations of rat gastric mucosa, small intestine and colon in vitro. Net secretory transport of all of the compounds tested was observed in the experiments with stomach. In the intestine the weaker bases were not transported but the more ionized compounds were secreted; in the colon, the stronger bases were transported in the absorptive direction. These transport processes were not dependent on transmural gradients of pH and in some cases the pattern of transport was maintained in the presence of adverse pH gradients. Studies on the effects of concentration and of interactions between concurrently transported compounds provided no evidence for carrier-mediated mechanisms. It was suggested that these observations are explicable in terms of a double membrane system.