Chemical induction of 6-thioguanine resistance was studied in synchronized human fibroblast cells. Cells initially grown in a medium lacking arginine and glutamine for 24 h ceased DNA synthesis and failed to enter the S phase. After introduction of complete medium, the cells progressed to the S phase after 16h. DNA synthesis peaked 20 h after removal of nutrient stress and declined. Mutations were induced in S-phase cells by methyl methanesulfonate (MMS), N-acetoxy-2-acetylaminofluorene (NA-AAF) and N-methyl-N'-nitro-N-nitroso-guanidine (MNNG). Chemical treatments resulted in an increase in the absolute number of mutant colonies and in a dose-dependent mutation frequency. In this report, we show that NA-AAF evokes a temporal pattern of mutation in synchronized cells, with such mutations being induced only during the S phase. Evidence indicates that presence of S-phase cells in the treated cultures is a prerequisite for the induction of mutations.