Abstract Adrenocorticotropic hormone (ACTH) stimulation tests were done in healthy and tumour-bearing dogs. In the tumour-bearing dogs, plasma endogenous ACTH (eACTH) concentration was measured and adrenal gland size was assessed ultrasonographically. Measurements in the tumour-bearing dogs were taken prior to therapy. No difference existed in basal or ACTH-stimulated cortisol concentration between tumour-bearing and healthy dogs. No difference existed in eACTH concentration between dogs with non-haematopoietic neoplasia (NHN) and lymphoma. However, of 20 dogs with lymphoma, 15% had increased basal serum cortisol concentration, 5% had an exaggerated response to ACTH and 5% had an increased eACTH concentration. Of 15 dogs with NHN, 20% had increased basal cortisol concentration, 7% had an exaggerated ACTH response and no dogs had an increased eACTH concentration. Of the dogs with lymphoma and NHN, 5 and 13%, respectively, had decreased basal cortisol concentrations; 20% of dogs with lymphoma and 13% with NHN had a subnormal ACTH response. eACTH levels were below the reference range in 10% of dogs with lymphoma and 7% with NHN. Overall, 10 adrenal glands were enlarged in seven dogs, five with lymphoma and two with NHN. The clinical significance of these findings remains to be determined.