AbstractFibroblast growth factor (Fgf) signaling plays important roles in development and metabolism. Mouse Fgf16 was predominantly expressed in cardiomyocytes. To elucidate the physiological roles of Fgf16, we generated Fgf16 knockout mice. Although the mice were apparently normal and fertile, heart weight and cardiomyocyte cell numbers were slightly decreased at 6 months of age. However, blood pressure, heart rate, and cardiac performance were essentially unchanged. In addition, the expression of most cardiac marker genes examined was also essentially unchanged. However, the expression of Bnp was significantly decreased, indicating potential roles of Fgf16 in the heart under pathological conditions. In contrast, the proliferation of embryonic cardiomyocytes was significantly decreased, indicating that Fgf16 is a growth factor for these cells. The embryonic heart phenotype is similar to that of the Fgf9 knockout heart, indicating Fgf9 and Fgf16 to synergistically act as growth factors for embryonic cardiomyocytes. Developmental Dynamics 237:2947–2954, 2008. © 2008 Wiley‐Liss, Inc.