The cDNA coding for the human renal isoform of the Na+/Ca2+ exchanger was cloned from a human embryonic kidney cell line (HEK 293) using a polymerase chain reaction strategy. It was found by sequence analysis that the cDNA encoded for a human kidney isoform of the Na+/Ca2+ exchanger. The kidney isoform is nearly identical to the human cardiac Na+/Ca2+ exchanger except for a segment of amino acids within the predicted cytoplasmic loop region of the molecule. Within this region, the kidney isoform contains a deletion of 36 amino acids, as well as a segment of 33 amino acids that is only 33% identical to the equivalent region of the cardiac exchanger. The cDNA was then tested for function by expression in insect cells. We constructed a recombinant baculovirus containing renal Na+/Ca2+ exchanger cDNA under control of the polyhedrin promoter. High levels of expression and Na+/Ca2+ exchange activity were found in Sf9 insect cells infected with the recombinant virus. Our results indicate that human kidney expresses a functional and alternatively spliced variant of the human cardiac Na+/Ca2+ exchanger.