The purpose of this study, which involved 134 children aged between 0.5 and 17 years, was to investigate the metabolism of isoniazid (INH) in children and to provide guide-lines for dosage. The distribution of slow (55,5%) and fast (44,5%) acetylators was the same in children as in adults. However, the mean of INH inactivation index were lower (0.23 and 0.48) while distribution volumes, (1.30 +/- 0.06 and 1.57 +/- 0.17 1/kg) and total plasma clearances (5.39 +/- 0.43 and 14.7 +/- 1.5 ml/Kg/min) were higher than in adults. Plasma half-lives were similar in children and adults. These results indicate that INH is more rapidly metabolized in children owing, it is suggested to a higher liver weight : body weight ratio and to a more pronounced first pass effect. The doses of INH therefore must be higher in children and strictly adapted to each individual.