Previous studies in vitro have shown that various mechanical methods used to wound plasma membranes allow normally impermeant, water-soluble markers, such as fluorescein dextran or horseradish peroxidase, to enter the cytosol. Subsequent membrane resealing traps these nontoxic fluorescent or electron microscopic markers within living, surviving wounded cells. The present report is the first, to our knowledge, to use this strategy to study cell membrane wounding and resealing in the intact animal. We show that gut cells wounded in vivo by mechanical forces are capable of resealing disruptions of their plasma membranes. More importantly, we show that wounding of cell membranes, followed by resealing, occurs not only in mechanically injured gut but also in normal, experimentally undisturbed gut. A variety of cell types were wounded and resealed membrane wounds in the mechanically injured stomach: surface mucous, endothelial, fibroblastic, parietal, and chief cells. Mucous cells successful at resealing membrane wounds apparently became active participants in the motile events of stomach repair. In undisturbed gut, cell membrane wounding and resealing was most frequently observed in the colon, but was also observed in the esophagus, stomach, duodenum, and ileum. Surface epithelial cells in undisturbed gut were retained for greater than 48 h after surviving membrane wounds. Two important roles are suggested for membrane resealing in gut: (a) preservation of motile cells nearest epithelial discontinuities requiring repair after injury, and (b) maintenance of epithelial integrity in normally functioning gut. Our finding of cell wounding in undisturbed gut may explain, in part, why rapid, continual cell turnover is characteristic of gut epithelia. We propose that membrane disruption, or wounding, is a normal and common occurrence in vivo, and that a biologically significant function of the plasma membrane is to reseal such wounds. The occurrence of in vivo cell membrane wounding and resealing suggests an unrecognized route for molecular traffic into and out of cytoplasm.