This paper presents an investigation into the effects of prolonged oestrogen treatment (20 days) on basal growth and on growth stimulated by GH in hypopituitary dwarf mice. Body weight and tail length were measured during the treatment period and uptake of 35SO4(2-) into costal cartilage in vivo at the end of the treatment period. This study confirmed that treatment with human GH promotes a dose-dependent increase in body weight, tail length and uptake of 35SO4(2-) in vivo; there was a highly significant correlation between these responses. Treatment with oestrogen alone had no significant effect on any of the parameters measured. All groups receiving combined oestrogen and human GH treatment showed a significant increase in body weight and tail length compared with animals receiving the same dose of oestrogen alone. However, the increase in body weight and tail length was significantly less in animals given the highest dose of oestrogen plus human GH than that observed in animals treated with the same dose of human GH alone. Treatment with oestrogen had no significant effect on the uptake of 35SO4(2-) stimulated by human GH. Possible mechanisms for the growth-inhibiting effects of oestrogens are discussed.