A high performance liquid chromatography (HPLC) assay was developed for the psychoactive cannabinoids Delta-9-Tetrahydrocannabinol (THC) and 11-Hydroxy-THC (11-OH-THC) using electrochemical detection (ECD). A C8 bonded column was used and the mobile phase consisted of acetonitrile, methanol and 0.01 M sulphuric acid at a flow rate of 1.5 mL/min. The detection limits for both THC and 11-OH-THC were 1.0 ng/mL. Preliminary screening of 193 drivers using an enzyme-multiplied immunoassay technique (EMIT) showed 21 tested positive on either blood, urine or both. Of these subjects 13 were confirmed as positive by the HPLC/ECD method in blood. Blood concentration for THC ranged from 1.4 ng/mL up to 20 ng/mL and for 11-OH-THC 2.5 ng/mL up to 85 ng/mL.