Histamine content and enzyme activities of histamine metabolism, histidine decarboxy-lase (HDC), histamine N-methyltransferase (HMT) and histaminase (diamine oxidase, DAO) in human nasal mucosa were determined with a highly sensitive and specific fluorescent method which was combined with high performance liquid chromatography. Histamine content and HDC activity were determined in 10 specimens of nasal polyp, nine specimens of maxillary sinus and five specimens of inferior turbinate. HMT and histaminase activities were determined in 15 specimens of nasal polyp, nine specimens of maxillary sinus and five specimens of inferior turbinate obtained during surgical therapy. Histamine and activities of HDC, HMT and histaminase were detected in all specimens except the case of histaminase activity in one specimen of nasal polyp. The mean values of histamine content and activities of HDC, HMT and histaminase of human nasal mucosa were 137.3 nmol/g wet weight, 26.3 fmol/min/mg protein, 26.4 pmol/min/mg protein and 0.5 pmol/min/mg protein, respectively. Histamine content in the mucosal tissue of the maxillary sinuses was significantly higher than that of nasal polyps or inferior turbinates. There were no significant differences in HDC activities among three kinds of nasal mucosa. Activities of HMT and histaminase, including their kinetic constants (Km and Vmax values for histamine) indicated that HMT has a greater potential than histaminase for histamine degradation in the human nasal mucosa. The presence of these enzymes suggests that these activities constitute an important modulating factor in histamine mediated allergic and inflammatory reactions in human nasal mucosa.