Both experimental colitis and human inflammatory bowel disease are characterized by an increased colonic blood flow. The objective of this study was to define the role of neutrophils in the colonic hyperemia associated with acetic acid-induced colitis in rats. One, two, and five days after the acetic acid enema, the colon was separated into five segments. Regional blood flow to each segment was measured using the radioactive microsphere technique. Tissue-associated myeloperoxidase activity was used as an index of neutrophil infiltration. Rectal blood flow and myeloperoxidase activity increased progressively after the acetic acid enema. At 5 days there were 3.9- and 4.6-fold increases in myeloperoxidase activity and blood flow, respectively. Comparable changes were noted in all bowel segments. The results suggest a temporal relationship between colonic blood flow and the extent of neutrophil infiltration. To assess directly the role of circulating and infiltrated neutrophils as mediators of the colitis-induced hyperemia, animals were rendered neutropenic approximately 8 h before the enema and neutropenia was maintained for another 24 h. Neutropenia did not modify the colitis-induced intestinal hyperemia normally observed at 24 h. We conclude from these findings that vasoactive agents derived from neutrophils do not mediate the increased colonic blood flow in this model of ulcerative colitis.