The succinic semialdehyde dehydrogenase (SSADH) inhibitory component was isolated from the EtOAc fraction of Lactuca sativa through repeated column chromatography; then, it was identified as phytol, a diterpenoid, based on the interpretation of several spectral data. Incubation of SSADH with the phytol results in a time-dependent loss of enzymatic activity, suggesting that enzyme modification is irreversible. The inactivation followed pseudo-first-order kinetics with the second-rate order constant of $6.15{ imes}10^{-2}mM^{-1}min^{-1}.$ Complete protection from inactivation was afforded by the coenzyme $NAD^{+}$, whereas substrate succinic semialdehyde failed to prevent the inactivation of the enzyme; therefore, it seems likely that phytol covalently binds at or near the active site of the enzyme. It is postulated that the phytol is able to elevate the neurotransmitter GABA levels in central nervous system through its inhibitory action on one of the GABA degradative enzymes, SSADH.