The initiation of cigarette smoking among adolescents is a health problem which has been the subject of discussion and study for many years. The evaluation of strategies to deter the adoption of smoking has long been hampered by the problems of measuring adolescent smoking behavior. Recently, interest has increased in biochemical measures of smoking under the assumption that they are more objective measures. The validity of this assumption is addressed for several ages of adolescents. This paper presents saliva thiocyanate levels, expired air carbon monoxide levels, and smoking self-reports from a sample of 2200 junior and senior high-school students. Interrelationships among the biochemical and behavioral measures are strong among the total population, ranging from 0.48 to 0.95 (Pearson r) but are much weaker at the younger age levels. Normative levels of carbon monoxide and saliva thiocyanate are presented by age (11-13, 14-15, and 16-17 years old). These data indicate that habitual smoking appears to develop in a gradual fashion and that several years may pass between initial experimentation and adult levels of smoking. Younger students consistently display lower levels of thiocyanate and carbon monoxide than older students of the same self-reported levels of smoking, suggesting that inhalation patterns may vary as a function of age or years smoking.