The gastrointestinal absorption of a series of vasopressin (VP) analogues with enhanced enzymatic stability was determined in chronically catheterized, conscious rats. The following peptides were used: [Mpa1,D-Arg8]vasopressin (dDAVP), [Mpa1,Asn4,D-Arg8]VP, [Mpa1,Val4,D-Arg8]VP, [Mpa1,(CH3)3Ala4,D-Arg8]VP, [Mpa1,Tyr(ethyl)2,D-Arg8]VP, and [Mpa1,D-Tyr(ethyl)2,Ile3,Val4,D-Arg8]VP. The peptides were administered by gavage feeding and blood samples were taken repeatedly for 3 h. In another series of experiments, plasma clearance rates (Clp) were determined using the constant infusion method. Plasma concentrations were measured by use of a cross-reacting dDAVP antiserum in a radioimmunoassay method. The bioavailability of all peptides was below 0.1%. The Clp values differed sevenfold; the lowest was for [Mpa1,D-Tyr(ethyl)2,Ile3,Val4,D-Arg8]VP and the highest was for [Mpa1,Asn4,D-Arg8]VP. With the exception of dDAVP the Clp values of the analogues showed an inverse relationship with hydrophilicity. Incubations in relatively concentrated intestinal contents for 1 h showed extensive degradation of the analogues except for [Mpa1,D-Tyr(ethyl)2,Ile3,Val4,D-Arg8]VP. It can be concluded that, in the rat, the bioavailability of dDAVP is lower than in other animal species and in man. Increased resistance to peptide degradation by gastrointestinal contents did not improve absorption. Therefore, the permeability properties of the intestinal mucosa are likely to be a more important factor affecting the gastrointestinal absorption of this group of peptides, although postabsorption events, like hepatic extraction, may also play a role.