The aim of this study was to evaluate the role of aldosterone as an initiating and/or perpetuating factor in hypertension associated with pituitary-dependent hyperadrenocorticism (PDH) in dogs. Thirteen dogs with PDH and 11 healthy control dogs were used. In all dogs, arterial blood pressure and plasma sodium, potassium, basal aldosterone, post-ACTH aldosterone, basal cortisol and post-ACTH cortisol concentrations were measured. The tests were repeated 10 days and three months after the beginning of o,p'-DDD treatment in PDH dogs. In untreated PDH dogs, plasma aldosterone was significantly decreased, whereas cortisol, sodium and arterial blood pressure were significantly increased compared to healthy dogs. Hypertension remained in most treated PDH dogs despite normalisation of cortisol and persistently low aldosterone levels. These results did not demonstrate that aldosterone is involved in the development and perpetuation of hypertension in PDH. However, glucocorticoids seemed to play a major role as an initiating and perpetuating factor in PDH in dogs.