Toads are chemically defended by bufadienolides, which are lethal to most predators. These toxins exert their lethal effects by binding to and disabling the Na+/K+-ATPases of cell membranes. Many species of snakes exhibit resistance to the effects of bufadienolides due to target-site insensitivity of the Na+/K+-ATPase. Mutations that confer resistance have previously been identified in ATP1a3, the gene that codes for the Na+/K+-ATPase α-3 paralog. We have found that this mutant gene is expressed at a significantly elevated level in heart tissue compared to gut, kidney, and liver of the bufadienolide-resistant snake, Thamnophis elegans. Furthermore, we found that exposure to bufadienolides elicits a significant increase in the expression levels of ATP1a3 in the heart, but not in the kidneys, liver, or gut 1 h after exposure. We suggest that upregulation of ATP1a3 in the heart plays an important role in the physiological processes involved in tolerance of bufadienolides among genetically resistant snakes.