Avascular segments of fetal rat intestine transplanted to the subcutaneous tissues of host syngeneic rats will become vascularized and grow. This study more fully characterizes this tissue, which we call "neogut," and compares it to normal rat small intestine. Anatomy was studied with light microscopy and scanning and transmission electron microscopy; transport and electrophysiologic parameters were measured in full-thickness pieces of tissue mounted in Ussing chambers; motility patterns, including slow wave and spike activity, were recorded. Subtle anatomic differences (shortened villi and microvilli) were noted in neogut compared to normal small bowel. Both neogut and normal rat ileum demonstrated net mucosal to serosal transport of d-glucose; the magnitudes of the electrophysiologic parameters (PD, Isc, and G) were less in neogut than in ileum. Slow-wave frequency of neogut was slightly less than native small bowel while spike activity was increased. These data show that neogut has structural and physiologic characteristics similar to normal rat small bowel and offers hope that this tissue may provide a nutritionally useful accessory gut for the patient with critical short-gut syndrome.