Sixty-seven cases of chemodectoma in dogs (fifty-nine aortic body, two glomus pulmonale, and six carotid body) were studied. The epidemiology, biological behaviour, and association with other tumours of canine and human chemodectoma were compared. The large number of sites of origin of human chemodectomas was not observed in the dog. The canine chemo-dectomas occurred as multiple tumours in 16% of the cases and were biologically similar to the human tumours. As with human chemodectomas, an association with endocrine tumours (such as phaeochromocyto-mas and thyroid neoplasms) was observed.

The aortic body tumours were the most common and occurred more frequently in males. A higher rate of metastasis (22%) and a higher frequency of multiple endocrine tumours in association with chemodectomas, not observed in the dog before, were seen. In contrast to the high frequency of interstitial cell tumours and a low frequency of Sertoli cell tumours noted in previous studies, a low frequency of interstitial cell tumours and a high frequency of Sertoli cell tumours were seen in this study. Similarly, a previously reported high frequency of haemangio-sarcoma was not observed in this series.

A significantly higher frequency of chemodectomas was seen in the brachiocephalic dogs (Boxers and Boston terriers). Chemodectomas in man have shown familial predispositions. However, in brachiocephalic dogs the high frequency of chemodectomas may be caused by their partially obstructed air passages, rather than by a genetic factor only.

A higher incidence of chemoreceptor-cell hyperplasia and chemo-