Biochemical determinations of plasma and salivary cotinine and thiocyanate were used to differentiate smokers from non-smokers and to follow daily smoking patterns in smokers. Results indicate that cotinine is better suited than thiocyanate to determine smoking status in large scale epidemiologic studies and to follow alterations in smoking behavior over periods of time. Salivary cotinine is a reliable alternative to plasma for validation of smoking status and for following changes in daily smoking patterns.