A sensitive and specific method was developed for the quantitative analysis of delta9-tetrahydrocannabinol (THC), 11 -hydroxy-THC (11-OH-THC), and 11-nor-9-carboxy-THC (THCCOOH) in human hair. Deuterated internal standards were added to hair samples, and samples were digested overnight in 1 N NaOH at 37 degrees C. Digest solutions were extracted with a liquid-liquid extraction procedure, which was previously developed in our laboratory for the analysis of plasma and whole blood. Derivatized extracts were analyzed on a Finnigan 4500" mass spectrometer in negative ion chemical ionization mode using methane as the reagent gas, hydrogen as the carrier gas, and a Restek Rtx 200-15M-0.25-microm capillary column. The assay was linear up to 50 ng/mg hair (r, 0.99) for all three compounds and was capable of detecting 10 pg THC and THCCOOH and 100 pg 11-OH-THC on column. The intra-assay precision ranged from 2.1 to 11.2% for the three analytes; the interassay precision ranged from 4.4 to 13.0%. The method was used to detect and quantitate the presence of THC, 11-OH-THC, and THCCOOH in human hair obtained from eight regular users of cannabis. THC, but not 11-OH-THC or THCCOOH, was detectable in the hair shaft above the assay limit of quantitation. Four laboratory wash procedures were also evaluated for their effect on the measured concentration of THC in hair. in seven of eight subjects, a methylene chloride wash procedure substantially reduced the measured THC concentration by up to 50%. The gas chromatographic-mass spectrometric assay is currently being used to support pharmacokinetic studies of drug disposition into the hair of humans and animals.