In order to examine the role of GH in the regulation of foetal adrenal development and function, we have localized GH-receptor mRNA and protein in adrenal glands of ovine foetuses at specific stages of gestation. Adrenals from 60-75 day (n=4), 100-110 day (n=4) and 140-145 day (n=3) foetal sheep (term is 145-150 days) and nonpregnant adult animals (n=3) were dissected and fixed. GH-receptor mRNA localization was studied by in situ hybridization using a 35 S-labelled antisense cRNA probe, and protein by immunohistochemistry using a specific monoclonal antibody to the GH-receptor. At all ages studied, GH-receptor mRNA and immunoreactivity could be detected throughout the adrenocortical region. In adult adrenals, GH-receptor mRNA and immunoreactivity were also evident throughout the adrenocortical zone, with the strongest expression confined to a defined region of cells at the interface between the zona glomerulosa and zona fasciculata. Northern blot analysis of 100 day and 140 day foetal adrenals confirmed the presence of a 4·4 kb GH-receptor mRNA transcript, while immunoblotting of foetal adrenals at approximately 110 days of gestation revealed a 55 kDa GH-receptor species. To study the effect of GH on the function and growth of the immature foetal adrenal gland in vivo, chronically catheterized ovine foetuses (n=4), between 100 and 110 days of gestation, were given a pulsatile infusion of recombinant bovine GH (125 µg/15 min, 24 pulses/24 h) for 72 h. Plasma cortisol and aldosterone levels were compared with age-matched controls receiving saline infusion alone (n=4). It was found that there was no difference in the basal plasma level of cortisol or aldosterone, and that infusion of GH did not alter steroid levels or gross adrenal size and morphology. These studies demonstrate strong expression of the GHreceptor in the developing ovine foetal adrenal cortex. However, in the immature foetus GH infusion is without effect on plasma steroid levels, suggesting that the steroidogenic action of GH in the ovine foetus may be gestationally dependent.