Abstract: Immunoblots of the soluble proteins from a rat brain high-speed supernatant dissociated under reducing conditions showed two monomers (molecular weights, 59,000 and 62,000 ± 2,000) immunolabeled by a glutamic acid decarboxylase (GAD) antiserum. In this extract, a GAD monoclonal antibody trapped the same two monomers, thus confirming that they are both constitutive sub-units of GAD. Without treatment under reducing conditions, two additional bands were stained by immunoblotting. Their molecular weights were estimated to be 115,000 and 122,000 ± 5,000. These results demonstrate the presence, in rat brain soluble extract, of two distinct forms of native GAD. They further support our previous hypothesis that each form is composed by the homodimeric association of each constitutive subunit through disulfide bridges.