Somatotropin treatment does not stimulate body growth in guinea-pigs, whereas it is effective in stimulation of both cytosolic and nuclear uterine ooestradiol receptor concentration in the animals fed optimum amounts of ascorbic acid. To determine whether this effect of somatotropin is ascorbic acid-dependent, guinea-pigs of 9 weeks of age with marginal vitamin C deficiency were treated with either recombinant bovine somatotropin (bST; 0.5 mg per animal) or vehicle for 10 days. The amount of available cytosolic ooestradiol receptor per unit of uterine weight, the DNA content, or in whole uteri was increased in somatotropin-treated animals (3.3 to 5.9- fold) compared to controls. However, the nuclear uterine oestradiol receptor concentration was not increased. The dissociation constant values were significantly higher in the cytosol (control: 2.79, bST-treated: 2.66) than in the nuclear fraction (control: 1.76, bST-treated: 1.80) and did not differ between control and bST treated animals. The results of this investigation demonstrate that guinea-pigs with marginal vitamin C deficiency provide a suitable model for studying the effect of vitamin C on somatotropin action. The possible synergistic action of ascorbic acid on uterine action of somatotropin is discussed.