This study has the purpose of investigating the influence of different high-fat experimental diets on myocardial structure in rats. Twenty-seven male rats were fed from 21 d old (postnatal age) until 18 mo old with one of the following supplemented diets: soy-bean oil (S) (n=6), canola oil (CA) (n=8), or lard and egg yolk (LE) (n=6) or canola oil+lard and egg yolk (CA+LE) (n=7). The blood pressure (BP) was measured, and after the sacrifice the cardiac biometry and the myocardial stereology were determined: cross-sectional area of cardiomyocyte (A), volume density (Vv), surface density (Sv), and length density (Lv) in relation to the cardiomyocytes (cm), connective tissue (ct), and blood vessels (v). The CA group rats had lower BP, A[cm], and Vv[ct]; they had'greater Vv[cm], Sv[cm], Vv[v], Lv[v], and Sv[v] than the other groups. The S rats had intermediary values for the myocardium and blood vessel parameters between the CA and LE group rats. These results support the notion that the long-term use of canola oil in the diet is better to preserve the myocardium structure, including microvascularization, than soybean oil or lard and egg yolk.