The objective of this work was to study the in situ and in vivo nasal absorption of paeonol. A novel single pass in situ nasal perfusion technique was applied to examine the rate and extent of nasal absorption of paeonol by rats. Various experimental conditions, such as perfusion rate, pH, osmotic pressure and drug concentration, were investigated. The in situ experiments showed that the nasal absorption of paeonol was not dependent on drug concentration, and fitted a first order process. The absorption rate constant, Ka, increased with an increase in perfusion speed. Paeonol was better absorbed in acidic solutions than in neutral or alkaline solutions. The value of Ka was higher in a hypertonic environment than under isotonic or hypotonic conditions. In vivo studies of paeonol absorption were carried out in rats and the pharmacokinetics parameters of intranasal (i.n.) and intragastric (i.g.) administration were compared with intravenous (i.v.) administration. The bioavailabilities of paeonol were 52.37% and 15.81% for i.n. and i.g, respectively, while T-max values were 3.05 +/-1.46 min and 6.30 +/- 0.70 min. MRT (Mean Residence Time) were 23.19 +/- 6.46 min, 41.49 +/- 2.96 min and 23.09 +/- 5.88 min for i.n., i.g. and i.v. methods, respectively. The results demonstrate that paeonol could be absorbed promptly and thoroughly by i.n. administration in rats.