Heat-stable enterotoxin (ST) producing Escherichia coli are a common cause of diarrhea in infants. ST acts through the stimulation of the guanylate cyclase-cGMP system. The effect of ST on the human intestine has not been investigated nor is any information available on the activity, distribution, or development of guanylate cyclase activity in the human intestine. The purpose of this study, therefore, was to characterize, these aspects of guanylate cyclase activity and to study the effect of ST on the activity and responsiveness of guanylate cyclase in the intestine of infants and children of various ages. We measured guanylate cyclase activity in 35 intestinal specimens, obtained operatively, from children aged 1 day to 16 yr. Guanylate cyclase activity was linear with protein concentration and time. Basal activity was similar in small intestine and in colon. In the small intestine, however, basal guanylate cyclase activity varied with age. It was maximal in children 1 day of age, and although somewhat variable, decreased with age thereafter. In colon, an age-related pattern was not found. E. coli ST stimulated guanylate cyclase activity in all specimens in a dose-related manner. In the small intestine ST-stimulation of guanylate cyclase was twice that found in colon. Furthermore, age affected the response of small intestinal guanylate cyclase to ST. Maximal response to ST was observed in children 1 day of age and ST stimulation was significantly greater in children less than 1 yr of age than in older children. In the colon, the response of guanylate cyclase to ST did not change with age. The greater responsiveness of small intestinal guanylate cyclase to ST in younger children may, in part, explain the severity of diarrhea induced by ST at that age. © 1987 International Pediatric Research Foundation, Inc.