The Ussing chamber technique was used as an oral absorption model for studies of the relative effects of the inhibition of enzymatic degradation and increased paracellular route on the transport of the poorly absorbed vasopressin analogues lysine vasopressin (LVP) and desmopressin (DDAVP). The rates of transport of LVP or DDAVP at 250 microM across ileum and colon segments were studied in the absence and in the presence of protease inhibitors (aprotinin and bestatin) and cytochalasin-B. During the different treatments, the rates of degradation of the peptides were also studied. Detectable amounts of LVP could only be measured on the serosal side of the intestinal segment in the presence of protease inhibitors or cytochalasin-B. The treatment with cytochalasin-B increased the rates of transport of both peptides severalfold, and the effect was reversible. We suggest that the Ussing chamber technique can be used to evaluate the reasons for low transport rates across intestinal membranes. The results also show that, apart from enzymatic degradation, the vasopressin analogues LVP and DDAVP have additional permeation problems; therefore, it may be necessary to increase the paracellular route to increase the absorption of these peptides.