Plant sources of omega-3 fatty acids (FA) are needed that can materially raise tissue levels of long-chain omega-3 FA [i.e., eicosapentaenoic acid (EPA; 20:5n-3) and docosahexaenoic acid (DHA; 20:6n-3)]. Stearidonic acid (SDA; 18:4n-3) is the delta-6 desaturase product of alpha-linolenic acid (ALA; 18:3n-3), and when fed to humans, increases red blood cell (RBC) content of EPA to a greater extent than does ALA. This study was undertaken to determine the dose-dependence and time course of the increase in the EPA and DHA content of the heart and RBC in dogs. Adult male Beagles were fed 21, 64, or 193 mg/kg of SDA in in their food daily for up to 12 weeks. Positive and negative controls were given EPA (43 mg/kg) or high oleic acid sunflower oil, respectively. The baseline EPA content of RBC was 0.38 +/- 0.03% which increased (P<0.01) in a dose-dependent manner, with the high dose of SDA and EPA achieving levels of 1.33 +/- 0.26 and 1.55. +/- 0.28%, respectively. In the heart, the content of EPA rose from 0.06 +/- 0.01 to 1.24 +/- 0.22% in the EPA group and to 0.81 +/- 0.32% in the high SDA group (both P<0.01). In both tissues, DHA did not change. Compared to dietary EPA, SDA was 20-23% as efficient in raising tissue EPA levels. In conclusion, SDA supplementation increased the EPA content of RBC and heart and may have utility as a plant-based source of omega-3 FA.