Atrial natriuretic peptide (ANP) is known to be present predominantly in the granules of adult cardiac atrial muscle cells. The functional role of this peptide is known to be involved in diuresis, natriuresis and control of blood pressure. The present study using antibody to ANP, revealed that the cultured adult cardiac ventricular muscle cells, unlike their counterparts in vivo, expressed ANP to be as abundant as in adult atrial cells. Cultured atrial cells did not express ANP any more than their counterparts in vivo. Both cells, specifically ventricular cells, contained various forms of ANP such as granular, aggregate, ring-shape, fibrous and amorphous in different parts of the cell body. Vesicular or granular ANP was also observed beneath the cell membrane, above the cell membrane or outside the cell body, which were possibly suggestive of exocytosis of ANP by cells. The current studies were extended to the cultured embryonic ventricular and atrial cardiac myocytes, and revealed that these cells expressed considerable ANP, which were comparable to those in in vivo cells, reported by others. The high expression of ANP in cultured adult ventricular cardiac muscle cells indicates that these adult cells are capable of producing high content of ANP as observed in embryonic ventricular cells.