Embryonic rat myocardium cultured in the anterior eye chamber of an adult rat increases in mass and differentiates into mature myocardium by most morphological criteria . When sympathetic innervation of grafted heart tissue was prevented by superior cervical ganglionectomy (SCGx), growth of the grafts was severely compromised. The present study used morphometric methods to examine the contribution of myocyte size to the differential growth of grafts in sympathetically denervated and intact eye chambers. For this purpose, atria or ventricles from 12-day gestation rat hearts were grafted into sympathetically denervated and intact eye chambers of male host rats. Tissue was harvested after either 2 or 8 weeks in oculo. Myocyte diameter and the ratio of cytoplasmic area to nuclear area increased between 2 and 8 weeks in oculo in both atrial and ventricular grafts. In these unloaded heart grafts, estimated myocyte size did not differ between atrial and ventricular myocytes. Grafts into sympathetically innervated and denervated eye chambers did not differ in myocyte diameter or in the ratio of cytoplasmic to nuclear area, suggesting that myocyte size cannot explain the smaller mass of grafts in sympathetically denervated eye chambers. Thus, it is likely that other factors such as myocyte proliferation, myocyte survival or altered tissue composition underlie the lesser growth of grafts not innervated by sympathetic nerves. © 1992.