The INH metabolism mainly involves acetylation to acetyloisoniazid by a non inducible hepatic enzyme N-acetyltransferase. Examples of drugs acetylated by this enzymatic system are: isoniazid, sulphadimidine. hydrallazine, dapson and sulphapyridine. The rate of acetylation is constant in any individual but varies in different patients. People are characterized as rapid or slow acetylators with slow acetylation inherited as an autosomal recessive. Heterozygotes have intermediate acetylation rate. The article presents comparative study of the bioavailability of INH in fast and slow acetylators in healthy volunteers. The results have shown that all compared parameters are significantly different in both groups of acetylators.