The mouse embryo cell line, BALB/3T3 clone A31-1-1, was used for quantitative studies on cytotoxicity, DNA damage and repair, mutagenicity and neoplastic transformation induced by different carcinogens. A ouabain resistance (oua-r) mutational assay was established for this cell line with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). Alkaline elution was used to study DNA damage and repair. Split-dose treatments with MNNG showed marked DNA repair during the intervals, but cytotoxicity, mutation and transformation frequencies were not significantly different after single or split doses of MNNG, regardless of the time interval between doses. In synchronized cell populations, although MNNG-induced mutation frequencies varied with cell cycle phases (maximum in S), transformation frequencies did not. Such dissociations in the cellular responses suggest different underlying mechanisms. The new phenomenon of temporal dissociation between exposure times required for maximal induction of mutation and transformation, previously observed in studies with MNNG at different initial concentrations, was confirmed and extended to ethylnitrosourea (ENU). The half-life of MMNG in the cultures was about 68 min. MNNG exposures required for maximal induction of DNA damage and mutation were only 30-60 min, but they were 120 min for cytotoxicity and 120-240 min for transformation; the ratio of transformation to mutation frequencies was 3.7 at short exposure times but increased to over 20 after 240 min or longer. DNA repair, as measured by alkaline elution, was active after 30-min exposures, but total DNA damage remained constant for MNNG exposures extended from 30 to 120 min. Equitoxic doses of ENU (half-life of about 12 min) induced maximal mutation in 5 min and transformation in 45-60 min. This temporal dissociation supports the hypothesis that transformation is dependent on factors other than a single gene mutation and offers a useful model for investigating the molecular events that occur during this differential time of exposure.