The recent demonstration of the ability of progestins to induce the expression of the growth hormone (GH) gene in the mammary gland of dogs and cats opens possibilities for the treatment of some forms of GH deficiency with progestins. Therefore, one male and one female German shepherd dog with congenital dwarfism because of a pituitary anomaly were treated with subcutaneous injections of medroxyprogesterone acetate (MPA) in doses of 2.5-5.0 mg per kg body weight, initially at 3-wk intervals and subsequently at 6-wk intervals. In both dogs, body sizes increased and a complete adult hair coat developed. Undesirable side-effects were recurrent periods of pruritic pyoderma in both dogs and cystic endometrial hyperplasia with mucometra in the female dog. Parallel with the physical improvements, plasma insulin-like growth factor I concentrations rose sharply. Plasma GH concentrations tended to rise, but never exceeded the upper limit of the reference range. Nevertheless, one of the dogs developed slight acromegalic features, possibly because mammary GH, unlike pituitary GH, is released evenly throughout the day. Even moderate increases in circulating GH concentration may, therefore, give rise to overexposure. It is concluded that long-term treatment with MPA can be used as an alternative for heterologous GH in the treatment of congenital GH deficiency in the dog.