The purpose of the present study was to determine whether myocardial oxygen consumption (MVO2) differs when the heart is arrested by hyperkalemic arresting solution (ASK) or by hypocalcemic arresting solution (ASCa) when coronary flow is maintained constant. MVO2 was measured in 12 isolated, Langendorff-perfused rat hearts alternately perfused with ASK (20 mM K+ and 1.5 mM Ca2+) and ASCa (5 mM K+ and 0.08 mM Ca2+). Six of the hearts were perfused with ASK for 10 min, ASCa for 5 min, ASK for a second 5 min, and finally ASCa for 5 min; ASCa and ASK were opposite in this sequence for the other six hearts. Measurements of MVO2 during ASK and ASCa arrest, taken at the end of each perfusion period, were analyzed to distinguish the independent influences of time and perfusate composition on MVO2 in the arrested hearts (analysis of covariance). Consistent with previous findings, MVO2 decreased with time after the onset of cardiac arrest with both solutions. The average per minute fall was 0.0003 ml O2.min-1.g-1 (P less than 0.01). However, at any given time after arrest, MVO2 averaged 0.004 ml.min-1.g-1 less during ASCa arrest than during ASK arrest (P less than 0.01), which amounted to a 15% reduction in MVO2. To test whether the increased MVO2 during hyperkalemic arrest was dependent on calcium in the perfusion medium, a third series of six hearts was studied in which MVO2 values measured during ASCa and ASK arrest were compared with those measured during arrest by hyperkalemic-hypocalcemic solution (ASK,Ca: 20 mM K+, 0.08 mM Ca2+).(ABSTRACT TRUNCATED AT 250 WORDS)