Nitrogen-containing organic compounds from environmental sources are receiving increasing attention because of uniquely active mutagens which have been found in this class (Chrisp et al., 1978; Nagao and Sugimura, 1978: Guerin et al., 1980) Differences in mutagenic activities among the various organo-nitrogen compounds, i.e., pyrrole types, pyridine types and aniline types, have been noted consistently. Furthermore, differences among homologs of a particular compound type are often striking. Information in this paper engages the question of chemical structure/biological activity relationships. Activity data for several N-heterocyclic, nitro-, amino- (primary, secondary and tertiary), and amino-N-heterocyclic aromatic compounds are presented. The number of fused rings and the substituent type affect the mutagenic activities greatly. The trends observed are discussed generally with reference to molecular structural features.