Pig brain contains three forms of glutamate decarboxylase with pI values of 5.3, 5.5 and 5.8, referred to as the alpha-, beta- and gamma-forms respectively. These forms were purified and kinetically characterized. The major synaptic form of glutamate decarboxylase (the beta-form) migrated as a single band on electrophoresis in sodium dodecyl sulphate/polyacrylamide gels with an apparent Mr of 60 000. Sodium dodecyl sulphate/polyacrylamide gel electrophoresis followed by immunoblotting with an affinity-purified antibody to the enzyme indicated a subunit Mr of 60 000 for the alpha- and gamma-forms as well. An extensive kinetic analysis, aided by an integrated equation that describes the inactivation and re-activation cycle of the enzyme, revealed that the three forms of the enzyme differ markedly in kinetic properties. The Km values for L-glutamate were 0.17, 0.45 and 1.24 mM respectively for the alpha-, beta- and gamma-forms. The Ki for 4-aminobutyrate, the first-order rate constants for inactivation by L-glutamate and 4-aminobutyrate, the rate constant for re-activation of the apoenzyme by pyridoxal 5'-phosphate and the dissociation constant for pyridoxal 5'-phosphate also differed in a similar way among the three forms; the values were in the order alpha-form less than beta-form less than gamma-form.