Diazoxide (DIAZ), an opener of mitochondrial ATP-sensitive K⁺ channels (mK_ATP), protects neurons against hypoxic/ischemic stress in vivo, however, direct evidence showing mitochondrial effects of DIAZ in postischemic neurons is lacking. We investigated if DIAZ affects mitochondrial alterations after global ischemia/reperfusion (I/R) in CA1 pyramidal neurons by using oxalate-pyroantimonate electron cytochemistry. Anesthetized piglets were either non-treated, or treated with DIAZ (3 mg/kg, iv), I/R, DIAZ+I/R, or 5-hydroxy-decanoate (5HD)+DIAZ+I/R (n=6, 6, 11, 5, 7, respectively). Ischemia (10 min) was induced by intracranial pressure (ICP) elevation. After 5–30 min of reperfusion, the brains were fixed for ultrastructural studies. Relative volumes of Ca²⁺-containing deposits and mitochondria in CA1 pyramidal cells were determined by point counting on electron micrographs. I/R resulted in maximal increases in mitochondrial volume (from 7.14±0.63% to 9.74±0.57%*), and Ca²⁺ levels (from 5.86±1.11% to 11.39±1.35%*; mean±S.E.M., *p<0.05) at 10–15-min reperfusion time. In this interval, pretreatment with DIAZ virtually abolished mitochondrial swelling (6.88±0.49%) and Ca²⁺ accumulation (5.15±0.82%) evoked by I/R. The protective effect of DIAZ was reduced by 5HD, an inhibitor of mK_ATP, resulting in a calcium accumulation similar to that after IR (10.44±1.98%). Thus, DIAZ might preserve mitochondrial integrity in CA1 pyramidal cells after I/R, at least in part mediated by mK_ATP.