Three types of chemically modified tetragastrin (TG) with fatty acids such as acetyl-TG, caproyl-TG and lauroyl-TG were synthesized and their in vitro intestinal permeability characteristics were estimated by a modified Ussing chamber system using the isolated intestinal membrane of rats. The penetration of TG across the large intestine was increased by conjugation with acetic acid and caproic acid but not lauric acid. Lauroyl-TG, a highly lipophilic derivative, exhibited low permeability across the intestinal membrane. A "bell-shaped" profile was observed between the apparent permeability coefficients and lipophilicity of the acyl-TG derivatives. The stability of acyl-TG derivatives was examined in homogenates of the jejunum, proximal and distal large intestine, liver and plasma. The half-lives for the proteolysis of the TGs were significantly prolonged by chemical modification with fatty acids in each homogenate. Thus, the chemical modification of TG with some fatty acids not only increases the lipophilicity of TG but also reduces its degradation, which resulted in increased intestinal absorption. The extent of the conjugates' hepatic first-pass metabolism was evaluated by gastric acid secretion activities after i.v. and intraportal administration. The amount of gastric acid secretion after intraportal administration of TG was significantly reduced in comparison with that after i.v. administration. On the other hand, conjugation with caproic acid slightly suppressed TG's hepatic first-pass metabolism, which suggests that chemically modified TGs with fatty acids would be more stable than the native TG in the systemic circulation after intestinal absorption.