In two dogs with hyperadrenocorticism due to an adrenocortical tumour, treatment with o,p'-DDD was started. Their hormonal response was monitored by measurements of the urinary corticoid/creatinine ratio. In one dog, two courses of 10 days treatment with o,p'-DDD were ineffective, whereas in the other dog the urinary corticoids decreased to very low levels after only six days of treatment, and corticosteroid supplementation had to be started. Two other dogs received o,p'-DDD according to a protocol used for the treatment of pituitary-dependent hyperadrenocorticism which aims at the complete destruction of the adrenal cortices, with substitution for the induced hyperadrenocorticism. Both dogs made a good recovery and their urinary corticoid/creatinine ratio decreased to within the reference range. In one of them the tumour had decreased considerably in size by five weeks after the start of the treatment.