Organic electrophiles have the potential to covalently attack DNA bases, and thus initiate mutagenic and carcinogenic processes. In this context, aromatic nitrogen sites of the DNA bases are often particularly nucleophilic, with guanine N7 being one of the most favored sites of adduct formation with electrophilic xenobiotics. Employing 4-(4-nitrobenzyl)pyridine (NBP) as model nucleophile with a respective aromatic ═N– unit, a new kinetic variant of a photometric chemoassay for sensing the DNA reactivity of organic compounds is introduced and applied to 21 three- and four-membered oxygen and sulfur heterocycles (15 epoxides, two thiiranes, three oxetanes, and one thietane). Besides six unreactive compounds (oxetanes, thietane, and aliphatic epoxides with six or more side-chain carbons), second-order rate constants of the electrophile-NBP reaction, k_NBP, were obtained for 15 compounds, ranging from (1.16 ± 0.05)·10^–3 to (36.5 ± 0.6)·10^–3 L mol^–1 min^–1 in a methanol/tris-HCl buffer (16/84 v/v) reaction medium. Solvolysis as confounding factor was addressed by determining respective first-order rate constants k_solv. Analysis of the k_NBP values resulted in structure–reactivity relationships, and comparison with literature data from the Ames test bacterial strains TA100, TA1535, and TA97 (Salmonella typhimurium) as well as from WP2 uvrA (Escherichia coli) revealed significant log–log relationships between the mutagenic potency of the heterocycles and their reactivity toward NBP. The latter demonstrates the potential of the NBP chemoassay as a nonanimal component of integrated testing strategies for REACH, enabling an efficient screening of organic electrophiles with respect to their DNA reactivity and associated mutagenicity and carcinogenicity.