The administration of oestrogens increases the hepatic synthesis and plasma level of ceruloplasmin both in man and laboratory animals. Methylene blue, an oxidizing agent and inhibitor of soluble guanylate cyclase, is widely used to block the effects of endothelium-derived relaxing factor (nitric oxide). We describe the inhibitory effect of methylene blue on the increase of ceruloplasmin plasma level in rats during oestradiol treatment.