In ribbed bivalve mollusc gills, the enzyme ketoglutarate dehydrogenase is regulated by pH, ketoglutarate and adenine nucleotides but not by calcium ions. In cauliflower, the enzyme is regulated by variations in Ca[superscript] 2+, Mg[superscript] 2+, adenine nucleotides and ketoglutarate. In plants, the decrease in mitochondrial Ca[superscript] 2+ inhibits KGDH whereas the entry of Ca[superscript] 2+ reactivates the enzyme. No such regulation appears to exist with the ribbed mussel KGDH.