It has been reported that degranulation of mast cells in rats, rabbits and dog was observed after dosing xylazine hydrochloride(Xh) which has been widely used as sedative, analgesic and muscular relaxant. Therefore, this experiment was conducted to examine the relations between Xh and histamine release and to identify the action of a-adrenoceptors which exists on the surface of mast cells. 1. The content of histamine within serum was measured with HPLC by performing the O-phthalaldehyde(OPA) fluorescent derivation. The pretreatment method had a little modification from the conventional method. The pretrement was carried out in the following method. 0.2㎖ of serum and 1㎖ of butanol were added to mixed together and then the liquid was centrifugally separated at 4℃ and 2,000rpm for 3 minutes. 0.4㎖ of 0.1N HC1 and 1.6㎖ of heptane were added to 0.8㎖ of supematant taken from the liquid, and they were mixed together. This mixture was also centrifugally separated at 4℃ and 2,000 rpm for 5 minutes. The supernatant was thrown away and the OPA fluo-rescent derivation was carried out with 0.2㎖ of the lower liquid then, 5 minutes after mixing 400㎕,a of 0.1N HC1, 120㎕ of 1N NaOH and 40㎕ of 0.1% OPA in the 0.2㎕ of the lower liquid, 120㎕ of 3.57N H3PO4 was added to the mixed liquid, and the liquid, was mixed again and syringe-filtered. Then, the measurement was done with HPLC in the 30:70 (v/v) ratio of 0.004M KH2PO4:CH3CN, flow rate of 1.0㎖/min., and a wavelength of λex=350nm and λem=444nm at the column temperature of 27℃, using the fluorescence detector. 2. The content of histamine in each laboratory animal appeared to be higher in such an order as rabbit, rat, guinea pig, dog, Korean indigenous goat, swine, Korean indigenous cattle, Holstein, and mouse, of which the individual mean values ± standard deviation were 2.0668±0.6049. 0.4999±0.2278, 0.4241±0.1974, 0.1054±0.0556, 0.1028 ±0.0276, 0.0972±0.0513, 0.0872±0.0373, 0.0717±0.0379, and 0.0706±0.0366, respectively. 3. The content of histamine was measured at the moments of 15-, 30-, 60-, 120-minutes after intramuscular injection of 20mg/100kg Xh into two to 4 years old Holstein weighing 600~700kg. The result showed that there was a significant increase at the times of 30- and 90-minutes after injection(p<0.05). 4. Intramuscular injection of 3mg/l0kg Xh was given to crossbred pug dogs weighing 2.5~4.3kg. The content of histamine was measured at the times of 30-, 60-, 90- and 120-minutes after injection. The result revealed that there was a significant increase at the times of 60~ and 90-minutes after injection(p<0.05). 5. Intramuscular injection of 10mg/㎖~25mg/㎖ Xh in concentration of 0.l㎖ was applied to Korean indigenous goat over 5 months old. Then, the content of histamine was measured at the times of 15-, 30-, 60- and 90-minutes after injection. A significant increase was shown at the times of 30- and 60-minutes after injection(p<0.06). 6. The content of histamine was measured at the moments of 30- and 60-minutes after intramuscular injection of 0.1-0.2㎖ Xh(20㎎/㎖) into male rabbits weighting 2.5-4kg. A significant increase was found at the moment of 60 minutes after injection(p<0.001). 7. After administering Xh to the mast cell taken from the abdominal cavity of mouse, the content of histamine was measured. The result showed that the higher the concentration, the more significantly the content of histamine was increased(p<0.05). 8. Compound 48/80 was administered in concentration of 5㎍/㎖ and 10㎍/㎖ to the mast cell picked from the abdominal cavity of mouse. The result showed that there was a significant increase in the content of histamine in case of the concentration of 10㎍/㎖(p<0.05). It was found to be about 10,000 to 500,000 times stronger than the Xh. 9. After premedication of 1mg/kg of yohimbine hydrochloride as a2-adrenergic antagonist to rabbits, the Xh was administered to them. The result was that the value of histamine within serum was decreased significantly(p<0.001). 10. After premedication of 1mg/kg of prazosin hydrochloride as a1-adrenergic antagonist to rabbits, the Xh was administered to them. It was found that the value of histamine within serum was decreased significantly(p<0.005). 11. Prazosin hydrochloride and yohimbine hydrochloride as a1-adrenergic antagonist, respectively, and a2-adrenergic antagonist were administerd. In this case, the value of histamine within serum was decreased significantly(p<0.0001). As the results, when the Xh is administered to various kinds of animals, the amount of histamine release within serum is increased. In view of the results so far achieved, it is concluded that Xh acted on both a1-adrenoreceptor and a2-adrenoreceptor induces the degranulation of mast cell.