The effect of the traditional Chinese herbal medicines (Tin Chuan Tang and Hsiao Ching Long Tang) on the serum concentrations and pharmacokinetics of aminophylline was examined in three different ages of SD rats. Each traditional Chinese herbal medicine was orally preadministered to SD rats for one week and then aminophylline was administered intravenously. The serum concentrations and pharmacokinetic parameters of theophylline were estimated by a two-compartment open model. The liver isolated after the last blood sampling was homogenized and the activity of hepatic cytochrome p-450 was determined. Significant difference was found in some pharmacokinetic parameters of theophylline such as K10, t 1/2, Cl and Vd for three different ages of SD rats without pretreatment with Chinese herbal drugs (p less than 0.05). However, pretreatment with Tin Chuan Tang or Hsiao Ching Long Tang did not affect the pharmacokinetic parameters of theophylline in three different ages of SD rats (p greater than 0.05). We also found that there was no correlation between age and activity of cytochrome p-450 of SD rats (p greater than 0.05). The decline in some pharmacokinetic parameters of theophylline in the elderly rats perhaps might be attributed to the decrease in hepatic blood flow and liver volume. It is concluded that there was no drug interaction between theophylline and Tin Chuan Tang or Hsiao Ching Long Tang in the different ages of SD rats.