Various extracellular-matrix proteins were detected by indirect immunofluorescence in rat intestine at various stages of development ranging from 14 days of gestation to the adult stage. At the earliest stage studied, laminin, nidogen and type-IV collagen were present at the epithelial/mesenchymal interface, whereas fibronectin and type-III procollagen were found throughout the whole mesenchyme. We were able to relate some changes in the staining patterns of extracellular-matrix proteins to morphogenetic processes. As early as 15 days of gestation, i.e. before villus formation, modifications in the distribution or in the staining intensity of all of the antigens within the mesenchyme paralleled the orientation and segregation of mesenchymal cells in the region surrounding the basal membrane and in the presumptive peripheral muscular layers. During villus outgrowth, the transient disappearance of fibronectin and particularly type-III procollagen from the top of the protruding villus core was evident. During the perinatal period, i.e. when crypts develop, the linear staining for the basal-membrane proteins became restricted to the base of the villi, their labelling along the remaining portion of the villi being more irregular. In mature rat intestine, no major modifications in matrix proteins along the crypt-villus axis in relation to epithelial differentiation were found, except that the labelling for fibronectin and type-III procollagen, which are at this stage more closely related to the basement membrane, was less pronounced in the upper part of villi.