Mineralocorticoid resistance (pseudohypoaldosteronism) is a rare condition first described in 1958 and associated with failure to thrive, salt wasting, and dehydration in infancy. In the index case it has previously been shown that binding of aldosterone to mineralocorticoid receptors in peripheral blood lymphocytes is absent; here, we report results of the molecular characterization of the mineralocorticoid receptor in this patient. Genomic DNA extracted from peripheral blood lymphocytes was subjected to Southern blot analysis after digestion with various restriction enzymes. There was no evidence of a major gene rearrangement or deletion. Oligonucleotide primers were designed on the basis of the published human complementary DNA sequence to cover the entire open reading frame of the mineralocorticoid receptor (MR). Total messenger ribonucleic acid (RNA) from lymphocytes was subjected to reverse transcription and amplification using the reverse transcriptase- polymerase chain reaction; the resulting fragments were then purified, subcloned, and sequenced. The patient showed no abnormality in the complementary DNA sequence corresponding to the open reading frame of the MR molecule compared with the published sequence. In addition, semiquantitative assessment of the patient's MR messenger RNA based on the reverse transcriptase-polymerase chain reaction technique suggested that he was producing MR RNA in roughly normal quantities. The mechanism of mineralocorticoid resistance in this case, therefore, remains uncertain, and the possibility must be considered that the underlying abnormality is not in the MR gene, but in an independent gene acting through yet to be characterized processes.