We prepared poly (A)+ mRNA from bovine adrenal medulla and electrophysiologically examined the expression of H1-histaminergic receptors in Xenopus laevis oocytes. Histamine dose-dependently induced inward currents in oocytes injected with the mRNA, but not in those injected with water or the non-injected oocytes. This response was also induced by H1-agonists and antagonized by H1-antagonists, while both an H2-agonist and an antagonist had no effect. These results clearly demonstrate that H1-histaminergic receptors are functionally expressed in oocytes injected with exogenous mRNA.