Endocrine cells were investigated in various parts of the gastrointestinal tract in an animal model for human type 2 diabetes, namely the homozygous obese mouse. As controls, age- and sex-matched homozygous lean mice were used. The different endocrine cell types were stained by using immunocytochemistry and quantified by computerized image analysis. The numbers of PYY- and enteroglucagon-immunoreactive (IR) cells were decreased in the colon of obese diabetic mice vis-à-vis controls. Serotonin-IR cells were significantly decreased in numbers in the duodenum and colon. Furthermore, colonic serotonin cells had a high cell secretory index (CSI), but an unchanged nuclear area, indicating an impaired cellular release of this amine. There was no statistical difference between obese diabetic mice and lean controls regarding the numbers and CSI of antral gastrin/CCK-, somatostatin- and serotonin-; and duodenal secretin-, gastric inhibitory peptide (GIP)-, CCK/gastrin-, and somatostatin-IR cells; nor was there any difference regarding nuclear area, with the exception of the antral somatostatin- and duodenal GIP-IR cells. It is concluded that the abnormalities in gut endocrine cells observed in the present study might explain the gut dysmotility reported in animal models for human diabetes. They may also be of importance in gastrointestinal dysfunction occurring in diabetes patients.