We have investigated the mutational specificities of two ethylating agents, N-nitroso-N,N-diethylamine (NDEA) and N-ethyl-N'-nitro-N-nitrosoguanidine (ENNG) in the NC+ domain of the lacI gene in Escherichia coli. G:C-->A:T transitions predominated in each of the two spectra, but its importance varies. While 95% of mutations induced by ENNG are G:C-->A:T transition, they account for only 61% of the total events following NDEA treatment. A:T-->G:C transitions are more often recovered following treatment with NDEA than with ENNG. Deletions, duplications, and frameshifts were also recovered after treatment with NDEA, but not ENNG. Mutations obtained in this study are also compared with those induced by corresponding methylating agents. The 5' flanking base appears to affect the distribution of the G:C-->A:T events. G:C-->A:T transition appears more likely to be recovered at 5'-PuG-3' sites. NDEA, however, induced a significant number of G:C-->A:T changes at 5-PuC-3' sites (51%). The possible reasons for these site-specificities are discussed. Although these alkylating agents most likely induce the bulk of their mutation via common mutagenic intermediates, each agent does induce a characteristic spectrum in which specific sites are enhanced or reduced.