P3a and P3b event-related brain potentials (ERPs) were elicited with an auditory three-stimulus (target, distracter, and standard) discrimination task in which subjects responded only to the target. Distracter stimuli consisted of white noise or novel sounds with stimulus characteristics perceptually matched. Target/standard discrimination difficulty was manipulated by varying target/standard pitch differences to produce relatively easy, medium, and hard tasks. Error rate and response time increased with increases in task difficulty. P3a was larger for the white noise compared to novel sounds, maximum over the central/parietal recording sites, and did not differ in size across difficulty levels. P3b was unaffected by distracter type, decreased as task difficulty increased, and maximum over the parietal recording sites. The findings indicate that P3a from white noise is robust and should be useful for applied studies as it removes stimulus novelty variability. Theoretical perspectives are discussed.