Mitochondrial NADP-specific isocitrate dehydrogenase (ICDH) partially purified from rabbit adrenals is sensitive to low concentrations of Ca2+. In the absence of Ca2+ (addition of 1 mM EGTA) the Km of ICDH for DL-isocitrate was 143 jaM, and in the presence of about 10 jaM Ca2+ (EGTA-Ca2+ buffer) it was decreased to 55 ¿iM. The JCm value of ICDH for NADP was similar in the absence (4.4 jxM) and in the presence (5.0 jiM) of Ca2+. The Km of mitochondrial NADP-ICDH for DL-isocitrate was higher than that of cytoplasmic NADP-ICDH from the same source.