Since highly concentrated NaCl is suspected to enter into the heart of the seawater eels, effects of high NaCl concentration on the atrial beating was examined, and plasma ion concentrations and osmolality were measured simultaneously in the blood collected from the bulbus arteriosus and from the caudal vessels. When 100 mmole l⁻¹ NaCl was added to the incubation medium, atrial contraction was enhanced significantly. Similar enhancement in the atrial contractility was also observed after addition of NaCH₃SO₄ (100 mmole l⁻¹) or Tris HCl (100 mmole l⁻¹), indicating that Na and Cl⁻ are not indispensable for the positive inotropic effect. Furthermore, an addition of sucrose (200 mmole l⁻¹) also enhanced the contraction. Inversely, hypoosmotic solution reduced the atrial contraction. These results indicate that the eel atrium is sensitive to environmental osmolarity. The eel atrium responses even at 20 mmole l⁻¹ sucrose. Such an inotropic effect of sucrose was not depressed after blocking adrenoceptor with betaxolol, a β₁-adrenoceptor antagonist, indicating that the effect is not due to adrenaline release from nerve endings. Plasma osmolality and Na concentration were higher in bulbus arteriosus than in caudal vessels, indicating that the eel heart is really exposed to hyperosmotic blood in sea water. The osmotically enhanced atrial contraction may increase the cardiac outflow into the gill. Such property of the atrium would have clear advantages for seawater teleosts, since the concentrated NaCl from the esophagus can be excreted immediately through the gill, without circulating their body, and blood homeostasis can be maintained efficiently.