In non-diabetic patients with severe disease, such as acute myocardial infarction or acute heart failure, admission blood glucose level is associated with their short-term and long-term mortality. We examined whether transient elevation of glucose affects contractile properties in non-diabetic hearts. Force, intracellular Ca²⁺ ([Ca²⁺]_i), and sarcomere length were measured in trabeculae from rat hearts. To assess contractile properties, maximum velocity of contraction (Max dF/dt) and minimum velocity of relaxation (Min dF/dt) were calculated. The ratio of phosphorylated troponin I (P-TnI) to troponin I (TnI) was measured. One hour after elevation of glucose from 150 to 400 mg/dL, developed force, Max dF/dt, and Min dF/dt were reduced without changes in [Ca²⁺]_i transients at 2.5 Hz stimulation and 2.0 mM [Ca²⁺]_o, while developed force and [Ca²⁺]_i transients showed no changes at 0.5 Hz stimulation and 0.7 mM [Ca²⁺]_o. In the presence of 1 μM KN-93, a Ca²⁺/calmodulin-dependent protein kinaseII (CaMKII) inhibitor, or 50 μM diazo-5-oxonorleucine, a l-glutamine-d-fructose-6-phosphate amidotransferase inhibitor, the reduction of contractile properties after elevation of glucose was suppressed. Furthermore, 1 h after elevation of glucose to 400 mg/dL at 2.0 mM [Ca²⁺]_o, the ratio of P-TnI to TnI was increased. These results suggest that in non-diabetic hearts under higher Ca²⁺-load, transient elevation of glucose for 1 h reduces contractile properties probably by activating CaMKII through O-GlcNAcylation. Thus, in the patients with severe disease, transient elevation of blood glucose, such as due to stress, may worsen cardiac function and thereby affect their mortality without known diabetes.