We analyzed the kinetics of mutation induction at the yg2 locus in Zea mays by N-ethyl-N-nitrosourea (ENU). ENU concentrations ranged from 0.1 to 10.7 mM and a linear, concentration-dependent response was observed for the induction of yg2 sectors in leaf 4 and leaf 5. For both leaf 4 and leaf 5 ENU exhibited one-hit kinetics consistent with either a point-mutational or chromosome-breakage mechanism of mutation induction. The mean induced mutation rate/mM ENU for the combined leaf 4 and leaf 5 data was 6.77 X 10(-4) mutations/locus/mM ENU. This value was used to compare the mutagenic effectiveness of ENU with ethyl methanesulfonate (EMS) and to calculate a rad equivalent for ENU. ENU was found to be 3.8-fold more effective than EMS. Based on their respective chemical reactivities with DNA, we propose that ENU predominantly induces point mutations at the yg2 locus. One rad of gamma radiation induced the same rate of mutation as an 8-hr exposure to a 6.71 X 10(-6) M concentration of ENU at 20 degrees C.