The aim of this study was to characterize the elimination half-life of delta 1-tetrahydrocannabinol in blood plasma in chronic marijuana users. The subjects smoked four cigarettes during a two day period, each cigarette containing 15 mg deuterium-labelled delta 1-tetrahydrocannabinol. The plasma concentrations of deuterium-labelled tetrahydrocannabinol were measured for 13 days using gas chromatography-mass spectrometry equipped with selected ion monitoring. The elimination half-life for delta 1-tetrahydrocannabinol in blood plasma was calculated to be 4.1 +/- 1.1 days (range 2.9-5.0 days) from the two week plasma level curves. Albeit the present results are based upon a small sample, an elimination half-life of delta 1-tetrahydrocannabinol in blood plasma of about 4 days is more in line with apparent half-life excretion of delta 1-tetrahydrocannabinol metabolites in the urine of chronic marijuana smokers.