Polyadenylated [poly(A)+] mRNA isolated from rabbit small-intestinal mucosa was injected into Xenopus laevis oocytes, and expression of the N2 Na+/nucleoside co-transporter was assayed by measuring Na(+)-dependent thymidine uptake. Expression of Na(+)-dependent thymidine uptake steadily increased after mRNA injection and was on average increased 11-fold by day 6 over background. Na(+)-dependent thymidine uptake was saturable (apparent Km approximately 30 microM at 22 degrees C) and inhibited by uridine and cytidine, but not by guanosine and inosine. These properties of the expressed thymidine transport strongly suggest that the epithelial N2 Na+/nucleoside co-transporter can be expressed in X. laevis oocytes.