Electrical coupling in the heart is mediated by gap junctions, aggregates of cell-to-cell channels composed of connexins. The principal cardiac gap-junctional connexin, connexin43 (Cx43), is reduced in diseased human myocardium that is prone to arrhythmia. Three additional connexin isoforms, Cx40, Cx45 and Cx37, of distinctive functional capacitiesin vitro, are expressed in cardiovascular cells, but our knowledge of their expression patterns in the human heart is fragmentary. In the present study, we therefore applied Northern blotting, Western blotting and immunoconfocal microscopy to analyse and compare the expression of Cx43, Cx40, Cx37 and Cx45 mRNA and protein in the human left ventricle, right ventricle, left atrium and right atrium of the human heart. Cx43 was confirmed to be abundantly expressed at similar levels by myocytes in all four chambers. Cx40 levels varied between chambers in the order right atrium >left atrium ≥ right ventricle ≊ left ventricle. Cx37 (exclusively expressed in the endothelium) was expressed at similar overall levels in all chambers (as judged from Northern blots). Cx45 was detectable only at very low levels, with a trend toward higher levels in the atria than the ventricles in a pattern similar to Cx40. The results indicate that in humans, the ventricles and atria have distinctive connexin expression profiles, and that the atrial-type connexin profile is more pronounced in the right atrium than the left atrium. While the ventricular connexin expression pattern resembles that of other mammalian species, atrial connexin expression shows greater species variation. These differences contribute to the interpretative framework for examining the potential role of altered connexin expression in ventricular and atrial arrhythmia in the human heart.