We studied the myocardium of 45 aged rats fed from 21 days after birth until 15 months of age with a standard rat diet a cholesterol-rich diet (CHO) or canola oil (O). We analysed the cardiac weight (CW) and, using unbiased stereological estimates, studied isotropic, uniform, random sections of the free left ventricular wall to determine the numerical density of the myocytes (NV[myocyte]). The CW was not statistically different between groups A and CHO: it was smallest in animals in group O (21.2% smaller in group O than in group A and 15.3% smaller in group O than in group CHO). NV[myocyte] was statistically different in all three groups and was greatest in animals in group O. By comparison with rats in group A, group CHO rats had an NV[myocyte] than was 51.3% smaller and group O, 33.3% greater. Aged rats fed with canola oil diet have a well-vascularized myocardium, which is probably associated with preservation of NV[myocyte] in the myocardium of these animals.