Abstract: An ultraviolet spectrophotometric method has been developed for indirect determination of aspirin (ASA)using the difference between total salicylic acid and free salicylic acid in rabbit plasma. Recoveries of salicylic acid were found to be above 98.4% and the method was linear from 8～1601μg with a correlation coefficient of 0.9999. The coefficient of variation was less than 3.85%.Using this method for ASA and Carlson's mcthod for nicotinic acid (NA), the plasma concentrations of these two orugs were cetermired when they were used iv alone or in combination in rabbits. The results analysed by a micro-computer were fitted to a two compartment open model with firstorder elimination. Among the pharmacekintic parameters of NS:, T_1/2(β) and AUC_0-∞ for the group (n=6) treated with NA 50 mg/kg in combination with ASA 50 mg/kg were 2.3374 h and 361.63 μg.h/ml, respectively, both significantly different from those for the group (n= 6) treated with NA 50 mg/kg alone which were 0.9846 h and 157.71μg·h/ml,resFectively(p<0.01). For ASA, no significant differences were demonstrated. The mechanism and significance of the prolonged T(1/2)(β) of NA were discussed.