It has been previously reported [1] that a two-fold increase in mechanical efficiency of the heart can be achieved following dietary supplementation of fish oil, namely DHA and EPA. In the same study, saturated fat-rich diet caused a significant decrease of efficiency. The objective of our study was to re-examine these two findings. Male Wistar rats, at the age of 6 weeks, were divided into three diet groups: reference (REF, n=15), fish oil-rich (FO, n=14) and saturated fatty acid-rich (SAT, n=16). The REF diet contained 6% fat (by weight), whereas both the high-fat groups (FO and SAT) contained 16% fat. Rats were fed the specified diet for 6-8 weeks, reaching weights of 400-500 g. At the end of this period, they were deeply anaesthetised with 5% isoflurane in oxygen and, after cervical-spinal dislocation, their hearts excised. Isolated working heart experiments were performed, as originally described by Neely et al. (1967) [2], at both 37 °C and 32 °C. The perfusate was a modified Tyrode’s solution, gased with 100% O2. Hearts were paced at 5 Hz and 4 Hz at 37 °C and 32 °C, respectively, and subjected to a range of preloads and afterloads. Pressure-volume work of the left ventricle, global myocardial oxygen consumption and mechanical efficiency were calculated. Analysis of Variance revealed no statistically significant effect of temperature on any energetic variable. More importantly, there was no difference (p = 0.96) of mechanical efficiency among the diet groups (9.6 ± 0.3, 7.3 ± 0.3 & 7.9 ± 0.2% at 37 °C and 8.7 ± 0.4, 10.6 ± 0.4 & 10.7 ± 0.2% at 32 °C, mean ± S.E.M., for REF, FO and SAT groups, respectively). We conclude that dietary supplementation with neither Omega-3-enriched fish oils nor saturated fat alters the mechanical efficiency of the myocardium.