A full assessment of all heart valves in rats and mice is often impractical and is usually not performed in routine toxicity studies, largely due to an inevitable inconsistency of histological sampling. The majority of reported heart valve changes involve the examination of a single, semirandom section through the heart and the valvulopathy occurring with age or induced by xenobiotics may have been generally underestimated in mice and rats. Here we describe the incidence and microscopic features of endocardial myxomatous change (EMC) in Hsd:S-D rats and CD-1 mice. EMC was common and widespread in both CD-1 mice and Hsd:S-D rats (188 of 220 rats and 96 of 215 mice were affected by EMC). Microscopically, EMC consisted of focal or segmental thickening of valves, primarily due to the presence of fibromyxoid tissue in the subendocardium. Occasionally, fibrin or thrombi deposits and collection of neutrophils or mononuclear cells were observed. These microscopic features were similar to those seen in valvular disease in humans induced by fenfluramine-phentermine (fen-phen), ergot alkaloids (ergotamine, methysergide), and carcinoid syndrome. The mitral valve in rats and pulmonary valve in mice were most frequently affected. An association between murine progressive cardiomyopathy (MPC) and EMC was noted only in rats, suggesting that there may be a possible relationship between MPC and EMC. However, additional research is needed to confirm a relationship between EMC and MPC in rats and/or mice.