The orientation of gulonolactone oxidase activity was investigated in rat liver microsomes. Ascorbate formation upon gulonolactone addition resulted in higher intravesicular than extravesicular ascorbate concentrations in native microsomal vesicles. The intraluminal ascorbate accumulation could be prevented or the accumulated ascorbate could be released by permeabilising the vesicles with the pore-forming alamethicin. The formation of the other product of the enzyme, hydrogen peroxide caused the preferential oxidation of intraluminal glutathione in glutathione-loaded microsomes. In conclusion, these results suggest that the orientation of the active site of gulonolactone oxidase is intraluminal and/or the enzyme releases its products towards the lumen of the endoplasmic reticulum.