We used light microscopy to compare the stereology of the myocytes from the following regions of the baboon's heart: a) cardiac walls (crista terminalis, interatrial and interventricular septa) and b) cardiac conduction tissue (atrioventricular bundle). Six hearts were perfused and several pieces of tissue were taken from these regions. A staggered test lattice consisting of cycloids was superimposed on twelve random pairs of photomicrographs in order to make disectors, and on twelve other random fields for each cardiac region in order to estimate certain stereological parameters. Statistically significant differences were found when comparing the number of myocytes from the crista terminalis (highest value) with either the interatrial or interventricular septum. There is also a significant difference between the number of myocytes in the interventricular septum and the atrioventricular bundle (highest value). For the mean profile area of the myocytes, significant differences were found between the myocytes of both the crista terminalis and the interatrial septum, and those of the interventricular septum (highest value). Under the light microscope, myocytes from the two regions of the atrial wall studied were indistinguishable in size, since they constitute a single cell population. It was also impossible to distinguish them from the atrioventricular cells. The myocytes from the interventricular septum differed in both size and number from those of the right atrium, but only in number from those of the atrioventricular bundle.