Structure maps are presented as an efficient means of indicating structure-reactivity relationships in metabolic pathway databases. The relative occurrence of N-demethylation and N-oxidation of N-methyl tertiary amines was examined using the structure map methodology. A new family of reaction site representations, the n-level representations, was developed to describe the N-methyl reaction sites of the compounds in the data set. It was possible to differentiate N-demethylation and N-oxidation reaction sites using a structure map constructed from a 3-level representation of the reaction sites.