Abstract: To study the effect of α₁-acid glycoprotein 1 (ORM1) polymorphism on the concentration of free nortriptyline in serum, genotyping analysis was employed in ORM1by sequencing. Eighteen unrelated male adults were chosen and given a single dose of 25 mg nortriptyline orally, then the blood samples were taken at 0, 1, 2, 3, 4, 6, 8, 12, 24, 32, 48, 72, 96 and 168 hours after drug administration. Nortriptyline and 10-OH-nortriptyline in serum and ultrafiltrate were detected for the total and free concentration by using HPLC-MS/MS. Pharmacokinetic parameters were compared among different ORM1 genotypes. No significant differences were shown in the pharmacokinetic parameters of total nortriptyline and 10-OH-nortriptyline. The mean AUC_0-∞ of free nortritpyline in ORM1*F1/*F1subjects was significantly higher than that in ORM1*F1/*S and ORM1*S/*S subjects ［(119.1±74.4) ng·mL^-1·h vs (51.4±23.2) ng·mL^-1·h and (42.4±11.6) ng·mL^-1·h］. The percentage of protein binding in subjects with ORM1*F1/*F1 genotype at 2, 3, 4, 6, 8 and 12 h after administration was slightly lower than in those with ORM1*F1/*S and ORM1*S/*Sgenotypes while the distinct difference was shown at 4 h (P<0.05). Different ORM1 genotypes might affect the protein binding percentage and the concentration of serum free nortriptyline. The ability binding to the drug was higher in subjects with ORM1*S/*S genotype than in those with other two genotypes, so as to cause the lower concentration of free nortriptyline.