A cDNA clone encoding a protein 69% identical in amino acid sequence with that of the Na/P(i) co-transporter NaP(i)-1 was isolated from a human kidney cDNA library. The DNA sequence was identical with that of NPT-1 cDNA published by Chong, Kristjansson, Zoghbi and Hughe (1993) (Genomics, 18, 355-359). In the present study, we have characterized the function of the encoded protein and the tissue distribution of its mRNA. Injection of RNA transcribed from NPT-1 into Xenopus oocytes resulted in expression of Na/P(i) co-transport activity showing a high affinity for P(i) transport (Km 0.29 mM). Kinetic characterization ([P(i)], [Na+]) demonstrated that the expressed transport activity has properties similar to those displayed by oocytes injected with human kidney poly(A)+ RNA. Northern blotting demonstrated that NPT-1 mRNA is expressed in renal cortex, liver and brain but not in other tissues. Hybrid depletion with antisense oligonucleotides to NaP(i)-3 and NPT-1 completely inhibited poly(A)+ RNA-induced Na(+)-dependent P(i) uptake in oocytes. These findings indicate that two high-affinity Na/P(i) cotransporters (NaP(i)-3 and NPT-1) are present in human kidney cortex.