In NZR/Gd inbred albino rats, tumours occurred in the right atrium or inferior vena cava of approximately 20 per cent. of untreated animals, of both sexes, over the age of 1 yr. The tumours were centred in the wall of the right atrium in 15 cases, and in the inferior vena cava in another three cases; they appeared to be primary in these sites. The tumours were slowly growing, but eventually malignant. Light- and electron-microscopic study showed the tumours were composed of epithelial alveoli imbedded in a collagenous connective tissue containing spindle cells and thin-walled blood capillaries. The epithelium varied from flat to low columnar, and often secreted mucoid material into the lumina, or sometimes into surrounding tissue. This tumour, for which the name atrio-caval epithelial mesothelioma is suggested, very closely resembles a rare epithelial tumour of the right atrium previously described in humans under a variety of names. An underlying embryological anomaly had been postulated in these tumours in humans, and the occurrence of pathologically similar lesions in high incidence in hearts of NZR/Gd inbred rats should help test the hypothesis of genetic and developmental causes in the genesis of this rare cardiac neoplasm.