The P300 (P3) event-related brain potential was elicited in a group of 50 children and young adults (4-20 years). A simple auditory task was employed in which subjects indicated with a finger movement when a randomly occurring target tone (high pitch) was presented in a series of standard (low pitch) tones. The probability of the target tone was varied across conditions at 10%, 30%, or 50% to assess possible developmental changes in P300 amplitude. Memory capacity was assessed with forward and reverse digit spans. Circumference of the head also was measured. P300 latency decreased with age, and P300 amplitude tended to become larger with age. Polynomial regression analyses revealed significant quadratic trends in these relationships, with changes leveling off for older subjects. Digit span and head circumference also were related curvilinearly to P300 values. Multiple regression analysis indicated that changes in age and memory span both predicted significant changes in P300 latency and amplitude. Target stimulus probability generally affected all subjects in a similar fashion, although the strength of the correlational relationships tended to decline with increases in probability. Developmental changes for the N1, P2, and N2 components from the standard stimuli also were obtained. The results are discussed in terms of previous P3 findings for children and their implications for future studies.