Huckel molecular orbital calculations and application of Seybold‐Smith reactivity theory suggests an explanation for the different carcinogenic potencies observed among selected aza‐polycyclic aromatic hydrocarbons in terms of the tendencies of these compounds to undergo specific metabolic activating reactions. Theoretical reactivity indices representing these reactions correlate with the carcinogenic activities of these environmental pollutants and can be used as an estimate of carcinogenicity in the absence of animal studies.