The present study is concerned with the absorption and disposition of a tripeptoid (N-substituted glycine derivative) and a tetrapeptide in the rat. The two compounds have similar backbone structures but differ with respect to the presence or absence of peptide bond. [ 3 H]tripeptoid and [ 3 H]tetrapeptide were administered orally (30 mg kg -1 ) and intravenously (i.v.) (30 or 3 mg kg -1 ) to Sprague Dawley rats. Blood, urine and feces were collected at designated times for radioactivity and parent drug analysis. The intestinal absorptive clearances of the tripeptoid and tetrapeptide were studied using an in situ rat intestinal perfusion model. The octanol/water partition coefficient of these two compounds was also determined. The results showed that the peptoid and peptide have similar absorptive clearance and octanol/water partitioning, but different in vivo absorption and disposition characteristics. The absorptive clearances of the tripeptoid and tetrapeptide were 6.7 and 4.8 Γ10 -4 mL min -1 cm -1 , respectively, and the corresponding octanol/water partition coefficients were 0.39 and 0.30. The extent of oral absorption of the tripeptoid was only 3 -8%, consistent with its low absorptive clearance. In contrast, the apparent absorption of the tetrapeptide was \ 75% of the radioactive dose. The peptide was completely metabolized within 2 h after an i.v. dose, whereas the peptoid was stable in blood and was primarily eliminated in feces as intact drug. In conclusion, the difference in in vivo absorption and disposition between the peptoid and peptide was apparently due to the presence or absence of a peptide bond. The tetrapeptide was subject to rapid metabolism in the body. Its relatively high absorption appeared to represent the absorption of metabolized radioactive fragments. The peptoid appears to have advantages over the peptide in term of metabolic stability, but its low oral absorption and rapid biliary excretion present additional challenges in the selection of an optimal drug candidate.