Compounds that inactive gamma-aminobutyric acid (GABA) transaminase have been shown to exhibit anticonvulsant activity. The mechanism of inactivation of GABA transaminase by 4-amino -5-fluoro-3-phenylpentanoic acid, gamma-vinyl GABA, gamma- ethynyl GABA, ethanolamine O-sulfate, (E)-4-amino-5-fluoro-2- pentenoic acid, (Z)-4-amino-2-fluoro-2-butenoic acid, and (Z)-4- amino-2-bromo-2-butenoic acid will be determined. (3H) Pyridoxal 5'-phosphate-reconstituted apo-GABA transamianse will be used to elucidate the structure of the inactivator adduct and to determine if an enamine mechanism is involved. A large scale synthesis of (3R, 4S)- and (3S, 4R)-4-amino-5-fluoro-3- phenylpentanoic acid will be carried out for use in several in vivo experiments. 2-Phenylisogabaculine will be synthesized, its inactivation of GABA transaminase studied, its specificity for the enzyme determined, and its in vivo effect examined. The mechanism of inactivation of GABA transaminase by beta- hydrazinopropionate will be investigated with the use of model studies and enzymatically. Experiments have been devised to determine why beta-halo GABA analogues are substrates, but not inactivators of GABA transaminase. This will involve the synthesis of new GABA analogues and the testing of their ability to inactivate the enzyme. A chemical model study for the enamine inactivation mechanism of PLP enzymes also is proposed. New mechanisms for inactivation of GABA transaminase by cycloserine will be tested. On mechanism, which involves aromatization of the inactivator, is used as the basis for the design of new mechanism-based inactivators of the enzyme. 4- Amino-2-difluoromethyl-2-butenoic acid will be synthesized as an activated form of the monofluoro analogue, and its inactivation properties studied. (R)- and (S)-4-Amino-2-hydroxymethyl- butanoic acids will be synthesized in order to determine the importance of the 2,3-double bond and the hydroxyl group in the potent inhibitor, (E)-4-Amino-2-hydroxymethyl-butanoic acids will be synthesized in order to determine the importance of the 2,3- double bond and the hydroxyl group in the potent inhibitor, (E)-4- amino-2-hydroxymethyl-2-butenoic acid. 1-(Aminomethyl) cyclopropanecarboxylic acid will be synthesized and tested as a mechanism-based inactivators of GABA transaminase, namely, 3- ethynyl-, 3-vinyl-, and 3-fluoromethyl-3-aminopropylsulfonate and -phosphonate, will be synthesized and tested.