Endothelin (ET) plays a pivotal role in the pathogenesis of cell growth disorders such as cancer. Atrasentan (ABT-627), a selective antagonist for the ET receptor A (ET A ), has shown benefit in controlling disease progression in men with hormone refractory prostate cancer who have undergone aggressive hormone ablation therapy. It is not known how hormone ablation affects ET-binding sites, although ET-1 and ET A expression are found to be elevated in prostate cancer patients. In this study, we examined the effect of castration on ET receptor binding in male beagle dogs. Three dogs were surgically castrated and another three sham-operated. The animals were sacrificed 1 week after operation and membranes were prepared from the prostate, heart, brain, kidney, liver and lung for ET-1, ET-3 and angiotensin II (A-II, as a control) binding studies. No significant difference in A-II binding was observed between castrated and sham-operated animals. However, ET-1 and ET-3 binding to prostate and brain membranes were altered significantly. From saturation binding studies using ET-1 in the prostate, the K d and B max values increased from 0.043nM and 0.094pmol/mg respectively in sham-operated dogs to 0.104nM and 0.311pmol/mg respectively in castrated animals. These results indicate that surgical castration in dogs produces a change in the ET receptor density in the prostate and brain, and may have implications for the effect of hormone ablation therapy on ET receptor expression in prostate cancer patients.