Escherichia coli, which produce heat-stable enterotoxin (ST(a)), cause intestinal fluid secretion as a mechanism of diarrhea. To determine the factors that modulate the intestinal secretory response, we first compared the time course of the ST(a)-induced secretion in ligated in situ loops of rat jejunum and ileum. We found that the jejunal secretory response was brief (≤30 min) while the ileal response ST(a) was sustained (≥3 h). We then compared the modification of purified ST(a) that occurred in jejunum and ileum and found a close correlation between the continued presence of unmodified, authentic ST(a) and continued fluid secretion in the ligated-loop model. At both sites alteration of ST(a) was demonstrated by high-performance liquid chromatography profile and brush-border membrane binding activity. However, in the jejunum, the modification of ST(a) was qualitatively different and quantitatively much greater. We conclude that the degree to which ST(a) is inactivated or removed from the intestine correlates with the secretory response observed. Inactivation of ST(a) may be a mechanism by which the host limits its secretory response.