AIM: Residual stress and strain are important for gastrointestinal function and relate to the geometric configuration, the loading conditions and the zero-stress state of the gastrointestinal tract. The purpose of this project is to provide morphometric data and residual strains for the rat small intestine (n=11). METHODS: To approach the no-load state. the intestine was surgically excised, transferred to an organ bath and cut transversely into short ring-shaped segments. Each ring was cut radially for obtaining the zero-stress state, The residual stress can be characterised by an opening angle. The strain difference between the zero-stress state and the no-load state is called residual strain. RESULTS: Large morphometric variations were found along the small intestine. The wall thickness was highest in the proximal duodenum and decreased in distal direction along the axis of the small intestine ( P < 0. 001). The circumferential length of the inner and outer surfaces decreased rapidly along the length of duodenum by 30-50 % ( P < 0.001). The wall area and lumen area showed a similar pattern ( P < 0.001). In zero-stress state the rings always opened up after making the cut, The experiments resulted in larger inner circumferential length and smaller outer circumferential length when compared to the no-load state, The well thickness and wall area did not differ between the no-load and zero-stress state. The opening angle and tangent rotation angle increased along the length of the duodenum and had its highest value 30 % down the intestine. Further down the intestine it decreased again ( P < 0.001). The serosal residual strain was tensile with the highest value close to the ligament of Treitz ( P < 0.001). The mucosal residual strain was compressive in all segments of the small intestine with average values between -0.25 and -0.4 and with the lowest values close to the ligament: of Treitz ( P < 0.001). CONCLUSION: Axial variation in morphometric properties and residual strains were found in the small intestine. Existence of large residual strains indicates that the zero-stress state must be considered in future biomechanical studies in the gastrointestinal tract.