A female dog (Collie dog, eight years of age, non-spayed) was referred to the University Clinic for Companion Animals with signs and symptoms suggesting endogenous progesterone-induced acromegaly and cystic endometrial hyperplasia. The dog had glucose intolerance, but the growth hormone concentration in plasma was within the reference range. The latter was probably due to the decline of progesterone at the end of the luteal phase, resulting in an abrogation of the process of progesterone-induced growth hormone hypersecretion. After ovariohysterectomy the glucose-tolerance normalized.