AbstractA new gas chromatographic method was developed for the quantification of levamisole in human plasma and urine, using a nitrogen–phosphorus flame ionization detector. The adsorption of the drug onto glass was prevented by treating the glassware with a siliconizing agent. The sensitivity of the assay was 10 ng ml−1and as low as 2 ng ml−1can be detected in plasma.The urinary metabolitep‐hydroxylevamisole was analysed by high performance liquid chromatography with ultraviolet detection. The sensitivity of this assay was 0·50 μg ml−1.Plasma and urinary concentrations of levamisole were determined in 10 healthy volunteers including seven men and three women following the administration of a single 150 mg dose of levamisole. Levamisole was rapidly absorbed (tmax1·5 h), giving a peak plasma concentration of 716·7 ± 217·5 ng ml−1. The plasma elimination half‐life of levamisole was 5·6 ± 2·5 h. Only 3·2 ± 2·9 per cent of the oral dose was recovered as unchanged drug in the urine, suggesting the importance of clearance of levamisole by routes other than the kidney, and most probably by hepatic metabolism.The urinary concentrations ofp‐hydroxylevamisole were determined before and after hydrolysis of the urine samples with β‐glucuronidase, and the level of conjugation of the metabolite with glucuronic acid was then estimated. Cumulative recovery of the metabolite accounted for 1·6 ± 1·1 per cent and 12·4 ± 5.5 per cent of the oral dose of levamisole before and after hydrolysis, respectively, indicating thatp‐hydroxylation is a relatively important route of metabolism of levamisole, and that thep‐hydroxylated metabolite is excreted mainly in conjugation with glucuronic acid.Except for the absorption rate of levamisole which is approximately twice as rapid in women as in men, there is no marked difference in the pharmacokinetics of l