In amphibian urinary bladder epithelium, vasopressin increases passive urea permeability, concomitant with the appearance of a facilitated urea transport. Amphibian oocytes from Xenopus laevis and Rana esculenta microinjected with total or fractionated poly(A +) RNA isolated from frog urinary bladder epithellal cells. After several (3-5) days at 18°C, the urea flux was assayed by measuring the uptake and efflux of [14C] urea in water-injected and mRNA-injected oocytes. A 2 to 3-fold increase of urea transport was detected in oocytes injected either with total mRNA or with a 6-10 kilobase mRNA fraction, when compared with water-injected oocytes. This expression of urea channels was inhibited by 0.1 mM phloretin (50% inhibition) and 0.1 mM nitrophenylthiourea (up to 70% inhibition). On the contrary, no expression was detected in brain mRNA-injected oocytes. These results show the specific functional expression of the phloretin and NPTU-sensitive urea channel (or carrier), from frog urinary bladder epithelial cells, providing an approach for the expression cloning of these urea channels.