Rats were subjected to 50% proximal or 50% distal small bowel resection or sham operation. Four weeks later intestinal absorption of copper was measured in vivo using a recirculation technique. In each rat 10 cm in situ segments of duodenum, midgut, and ileum were perfused through the lumen for 2 h with 15 ml of an isotonic solution containing 2 micrograms/ml copper together with 14C-polyethylene glycol as volume marker. Absorption was expressed per g dry mucosa and per cm segment length. After proximal enterectomy absorption of copper per g mucosa decreased in the duodenum and midgut but remained unchanged in the ileum compared to sham-operated controls. However, there were adaptive increases in mucosal mass in all segments. Copper absorption per cm segment length in duodenum and midgut remained unchanged from controls and ileal absorption per cm increased in proportion to mucosal growth. After distal enterectomy absorption of copper per g mucosa remained unchanged in the duodenum but decreased in the midgut and ileum compared to sham-operated controls. Adaptive mucosal growth now occurred only in the midgut and ileum. In the ileum mucosal mass more than doubled. Copper absorption per cm segment length in the duodenum and midgut did not change from controls. However, in the ileum absorption per cm increased above that of controls because of the marked mucosal growth. Therefore, in response to either proximal or distal small bowel resection, the ileum was the site for adaptive copper absorption but adaptive mechanisms differed.(ABSTRACT TRUNCATED AT 250 WORDS)