Cotinine is a major metabolite of nicotine in man. Its disposition kinetics has been followed in plasma and saliva from nine nonsmokers, 23 to 56 years of age. Cotinine 5, 10 and 20 mg was given intravenously and orally to each subject, and plasma, saliva and urine samples were collected for 96 h. The kinetics of cotinine was best described by a multicompartment model with three distinct phases both in plasma and saliva. Regardless of the mode of administration, there was no indication of dose-dependent kinetics. Mean total plasma clearance was 63.8 ml.h-1.kg-1 and mean renal clearance was 4.7 ml.h-1.kg-1, i.e. only 10% of the dose was excreted unchanged in the urine. The volume of distribution, as calculated from the plasma curves, was slightly greater than the body weight, 1.1 l.kg-1. The concentration of cotinine was 20 to 40% higher in unstimulated mixed saliva than in plasma during the absorption, distribution and elimination phases. As the clearance and distribution values in saliva were directly proportional to the corresponding values in plasma, similar terminal half-life values were obtained in the two body fluids, 15.5 and 16.8 h for plasma and saliva, respectively. Thus the kinetics of cotinine is linear after intravenous and after oral dosing, and salivary concentrations give the same information about cotinine disposition in the body as do plasma concentrations.