A hair root was evaluated as a specimen for proving acute methamphetamine (MA) poisonings using an animal model and fatal cases of MA intoxicaton. First of all, male pigmented hairy rats (n = 5) were administered with acute poisonous doses (20, 40 and 60 mg/kg) of MA and the hair roots were plucked out with a hair nipper 5 min and 0.5, 1, 2, 6 and 24 h after i.p. injection. The hair root samples were, directly or after washing with detergent, extracted with methanol/5 N HCl (20:1) under vortex mixing at room temperature for 14 h. After evaporation, the residue was derivatized with pentafluoropropionic anhydride and analyzed with GC/MS. From all samples including a 5-min sample, MA was detected at high concentrations (approximately 150 ng/mg) with a small amount of amphetamine (AP). Many animals died within 120 min of administration, but the concentrations in the hair roots increased up to 120 min and then slowly decreased until 24 h. Although MA was definitely detected anytime in the hair roots, almost no MA was found in 24-h plasma. In comparison of the drug levels in hair roots between the washed group and the unwashed group, the levels of the washed group were as a whole 4-5-fold higher than those of the unwashed group. These differences show that most of the drug incorporated into hair root is still not immobilized in the early stage. The ratios of the MA remainder in the washed samples increased with the elapse of time in all cases. However, the slope of the curves definitely dropped after the death of rats, probably due to the stopping of the hair growth and the incorporation of drug into the hair shaft. The ratios of AP/MA after death became a plateau probably due to the stoppage of the activity of metabolism after death, while those before death had increased over time. We analyzed the specimens of hair root of four men who died mainly due to acute poisonings with MA. Consequently, MA in the hair roots was detected at high concentrations, 30.5-134.6 ng/mg, and its metabolic, AP, at the concentrations of 1.2-9.0 ng/mg. Our results suggested that hair root is a good specimen for probing acute MA poisoning.