In our search for the anticonvulsant constituent of Gastrodia elata repeated column chromatographies guided by activity assay led to isolation of an active compound, which was identified as gastrodin on the basis of spectral data. Brain succinic semialdehyde dehydrogenase (SSADH) was inactivated by preincubation with gastrodin in a time-dependent manner and the reaction was monitored by absorption and fluorescence spectroscopic methods. The inactivation followed pseudo-first-order kinetics with the second-rate order constant of 1.2 x 10(3) M-1min-1. The time course of the reaction was significantly affected by the coenzyme NAD+, which affected complete protection against the loss of the catalytic activity, whereas substrate succinic semialdehyde failed to prevent the inactivation of the enzyme. It is postulated that the gastrodin is able to elevate the neurotransmitter GABA levels in central nervous system by inhibitory action on one of the GABA degradative enzymes, SSADH.