The present study tested the hypothesis that a reduction in calcium flux across the sarcolemma or the sarcoplasmic reticulum at the onset of reperfusion could attenuate subsequent mechanical “stunning” (postischemic myocardial dysfunction). The isolated working rat heart was subjected to 20 minutes of total global ischemia, reperfused in the Langendorff mode for 5 minutes, and then made to work again for 10 minutes. During the early reperfusion period (first 2 minutes), the effects of agents thought to increase cytosolic calcium (high external calcium [modified Tyrode's solution replaced Krebs-Henseleit buffer as the perfusatel, isoproterenol, forskolin, and Bay K 8644) were tested. All these interventions worsened stunning. The cardiac output (CO) of control hearts recovered to 74.7 ± 3.4%, whereas recovery was 56.3 ± 3.7% (p<0.05) for high calcium (10 mM), 53.4 ± 3.6% (p<0.05) for isoproterenol, 43.4 ± 4.1% (p<0.05) for Bay K 8644, and 62.7 ± 2.4% (p<0.002) for forskolin. Interventions aimed at limiting calcium flux during early reperfusion, such as reperfusion with a low extracellular calcium or the addition of ryanodine (3 × 10–9M), nisoldipine (10–8M), or the inorganic blockers Mn2+(2 mM) or Mg2+(16 mM), were also tested. Low extracellular calcium (0.75 mM) improved CO to 91.8 ± 0.8% (p<0.05). Reperfusion with ryanodine and nisoldipine gave CO recoveries of 103.6 ± 1.8% (p<0.002) and 99.0 ± 2.8% (p<0.002), respectively. The addition of Mn2+and Mg2+resulted in CO recoveries of 88.9 ± 2.4% (p<0.05) and 91.9 ± 1.4% (p<0.002), respectively. Ryanodine and nisoldipine pretreatment changed CO recoveries to 97.5 ± 1.8% (p<0.002) and 84.8 ± 5.2% (p=NS), respectively. In conclusion, the use of organic or inorganic calcium antagonists or ryanodine at the onset of reperfusion attenuated reperfusion stunning in the isolated rat heart. Agents thought to promote calcium influx and/or to increase cytosolic calcium levels exaggerated the severity of stunning. These data support the hypothesis that the development of myocardial mechanical stunning can be related to cytosolic calcium overload at the onset of reperfusion. (Circulation Research1992;70:960–967)