VIGABATRIN, R-(-)- is an inactive enantiomer of gamma-vinyl-gamma-aminobutyric acid. Vigabatrin exists as a 50/50 racemic mixture of two enantiomers. It is an irreversible inhibitor of the GABA-transaminase, the enzyme responsible for the catabolism of GABA, thus increasing the GABAergic transmission. This drug is mainly prescribed for the treatment of West syndrome, a deleterious pediatric epileptic syndrome also known as infantile spasms. The maximum and minimum plasma concentrations of vigabatrin at steady-state were lower for the S(+) than for the R(-) enantiomer, while the apparent oral clearance was higher for the S(+) than for the R(-) enantiomer in a patient affected with tuberous sclerosis who developed major agitation and aggression and in whom impaired renal function was diagnosed. The question remains of the potential toxicity of the high levels of the R(-) enantiomer. The placental uptake of the active S(+)-isomer from the maternal circulation exceeded that of the R(-)-isomer and this was reflected by a corresponding difference in placental tissue concentrations.