OBJECTIVE To assess the effects of ischemic postconditioning on left ventricular function in isolated rat hearts. METHODS Twenty-four Wistar rats were used. These hearts underwent perfusion by modified LANGERDORFF method and distributed into three groups: GI--control (n=8); GII--three cycles of postconditioning of 10/10s (n=8); GIII three cycles of postconditioning of 30/30s (n=8). After a 15 min stabilization period, all hearts underwent 20 min of global ischemia following 20 min of reperfusion. In the times 0 (control), 5, 10, 15 and 20 min of reperfusion, the heart rate (HR), the coronary flow (CoF), the systolic pressure, the (+dP/dt max) contractility and (-dP/dt max) velocity of relaxation were measured. Data were analyzed by ANOVA method followed by Tukey's test for differences between groups and P < 0.05 was considered significant. RESULTS The HR (bpm) decreased in all groups after 20 min of reperfusion without statistical differences among them (GI 232.5+36.8; GII 241.8+46.7; GIII 249.4+40.4;P>0.05). The same occurred with the systolic pressure (mmHg) (GI 132.6+49.3; GII 140.8+43.1; GIII 112.6+33.2; P>0.05), coronary blood flow (GI 18.5+4.6; GII 21.4+4.4; GIII 22.1+9.0; P>0.05) and -dP/dt max (mmHg) (GI 1490.6+512.0; GII 1770.4+406.6; GIII 1399.1+327.4; P>0.05). The +dP/dt max (mmHg) decreased significantly in all groups except in group II (GI 1409.0+415.2, GII 1917.3+403.1, GIII 1344.8+355.8) (GII vs GI, P= 0.04; GII vs GIII, P= 0.02). CONCLUSIONS The ischemic postconditioning by three cycles of reperfusion/ischemia of 10/10s demonstrated to be effective for preservation of the myocardial contractility in isolated rat hearts which had undergone 20 min of ischemia.