The cardiac sarcolemmal Na(+)-Ca2+ exchanger is the primary mechanism for extrusion of calcium from the cardiac myocyte and therefore is important in regulating cardiac contractility. As part of an effort to determine whether the exchanger is associated with any genetic disorders of the heart or blood pressure, we have assigned the exchanger gene (designated NCX1) to human chromosome 2p21-p23 by analysis of a panel of mouse-human somatic cell hybrids and by in situ hybridization.