Rapid growth of a transplantable insulinoma in rats over a 14 day period results in hyperphagia together with a selective increase in the weight and enteroglucagon (GLI) concentration of the small intestine. Measurement of serotonin concentrations by an HPLC/fluorometric method demonstrated an increase in blood (91%; p less than 0.05) and in extracts of colon (22%; p less than 0.05) but no change in extracts of the small intestine and caecum. The data support the hypothesis that the rise in enteroglucagon is related directly to the growth of the small intestine and is not a non-specific effect of hyperphagia.