Xenopus laevis oocytes have been used for the expression of a renal, pyrimidine-selective, Na(+)-nucleoside cotransporter (N2). As compared to its uptake in water-injected oocytes, Na(+)-dependent thymidine uptake was enhanced in a time- and dose-dependent manner in oocytes injected with rat renal cortex total poly(A)+ RNA. An increased uptake was also observed after injection of size fractionated rat renal cortex poly(A)+ RNA (2-3 kb). Consistent with the selectivity of the N2 nucleoside transporter, cytidine significantly inhibited Na(+)-dependent thymidine uptake in oocytes injected with total poly(A)+ RNA whereas guanosine and formycin B did not. Na(+)-dependent thymidine uptake was also enhanced in oocytes injected with size fractionated human renal cortex poly(A)+ RNA (2-3 kb). The above data demonstrate functional expression of renal cortex, Na(+)-nucleoside cotransporters in Xenopus laevis oocytes.