In a study on the effects of smoked cannabis (18.2 +/- 2.8 mg as low and 36.5 +/- 5.6 mg as high dose) paired blood and oral fluid samples were collected from 10 study participants up to 6 h after smoking and analyzed for the cannabinoids Delta(9)-tetrahydrocannabinol (THC), 11-hydroxy-THC (THC-OH) and 11-nor-9-carboxy-THC (THCA) using gas chromatography-mass spectrometry. Highest concentrations in serum were 47.8 +/- 35.0 and 79.1 +/- 42.5 microg/L at the end of smoking (low and high dose, respectively) and decreased to less than 1 microg/L during 6 h with elimination half-lives of 1.4 +/- 0.1 h calculated from 1 to 6 h, which is shorter than reported previously. The elimination half-lives of THC-OH (2.0 +/- 0.3 h) and THCA (3.4 +/- 0.9 h) were significantly higher. The THC concentrations in oral fluid were highest with 900 +/- 589 and 1041 +/- 652 microg/L (low and high dose, respectively) in the first sample collected at 0.25 h and decreased to 18 +/- 12 microg/L over 6 h with elimination half-lives of 1.5 +/- 0.6 h. The elimination half-life of THC in serum and oral fluid and between the two doses did not significantly differ. Oral fluid/serum ratios were 46 +/- 27 and 36 +/- 20 (low and high dose, respectively), which are higher than previously reported and might be based on sample collection and/or analytical issues. In conclusion, despite similar elimination rates of THC in serum and oral fluid, which appear incidental, the high differences in oral fluid/serum ratios are not a reliable basis for correlating THC concentrations in oral fluid and serum. The oral compartment and its kinetics for drugs, particularly THC, are not yet satisfactorily understood.