Dissolution and absorption rates (in vitro); clearance from blood and elimination rates of N-nitrosoephedrine (NEP) and N-nitrosopseudophedrine (NPEP) in mice were determined. The two isomers obeyed first-order kinetics and from the slope of the regression line, the rate constant for each study was obtained. These constants were 0.023, 0.038 min-1 (dissolution); 0.192, 0.225 h-1 (stomach absorption); 2.898, 1.980 h-1 (intestinal absorption); 0.33, 0.55 h-1 (blood clearance) and 0.373, 0.393 h-1 (elimination from whole animal) of NEP and NPEP, respectively, and were tested by Student's t-test. Significant differences in the dissolution, absorption and blood clearance rates of NPEP from those of NEP have been observed (P less than 0.05-0.01). These differences are expected to exert their influence on the metabolic rates and the carcinogenic and/or spectral properties of NEP and NPEP, in a related pattern.