Affective stimulus pictures that differed in valence (unpleasant, neutral, and pleasant) were repeated as targets in an oddball task to elicit event-related potentials (ERPs) in young female adults. Each picture target was repeated consecutively four times, with picture order counterbalanced and time-on-task influences assessed across subjects. Response time decreased from the first to second stimulus presentation and remained stable. Stimulus repetition was associated with voltage increases for N1, P2, N2, and P3, from initial to subsequent presentations. Arousal effects did not interact with stimulus repetition at any latency range. Time-on-task was associated with decreased voltages for the N2 and P3 potentials but was unaffected by stimulus valence. The findings suggest affective arousal, stimulus repetition, and time-on-task independently modulate ERP outcomes at overlapping time ranges. Theoretical implications are discussed.