Seven cases of dog islet cell carcinomas were studied by conventional and immunohistochemical light- and electron-microscopy. Antisera to insulin, pancreatic polypeptide, somatostatin and glucagon were used. In 6 tumours several hormones were demonstrated. Glucagon never occurred. Insulin was the only hormone present in every tumour, thus it seems to be a good marker for these neoplasms. Liver metastases contained less immunoreactive cells than primary tumours and cell types found in primary carcinomas were sometimes not present in liver metastases. In two cases a degenerative neuropathy occurred.