The isolation and gas chromatographic quantitation of 8-methoxypsoralen (8-MOP) is described. The method involves extraction of the drug from serum, thin layer chromatography of the extract and gas liquid chromatography of the eluate from the thin layer plate. Serum concentrations of 8-methoxypsoralen after oral administration have been determined. The drug concentrations were determined by gas chromatography or after administration of tritiated drug, by measuring the radioactivity in serum. Using the gas chromatographic method, maximal serum levels of 0.4-1.5 gg 8-MOP per ml were found 70 -90 rain after oral administration of 50 mg of the drug. Measuring tritium levels in serum, maximal concentration of 0.9 gg 8-MOP per ml was calculated, reached 40 min after administration of 20 mg of tritium labelled 8-MOP. Measuring radioactivity, the sensitivity is very high, but the method does not distinguish between the original drug and its metabolic products. The gas chromatographic method, on the other hand, detects the original drug and the results are not affected by the presence of breakdown products. Rapid absorption of the drug from the gastro-intestinal tract was observed following oral administration. The drug is rapidly metabolised and excreted from the body, showing an apparent half life time of about 1.5 h. No detectable drug could be found in serum 10 h after administration when the GLC method was used; however, excretion of radioactive metabolites continued over several days and may indicate a possible accumulation of metabolites, especially in cases when a frequent 8-MOP treatment schedule is applied.