This study investigates the possibility of measuring cognitive responses to musical stimuli. The experiments were based on measurements of the event- related potential (ERP) of three electroencephalographic electrodes. The musical stimuli consisted of five-tone pitch patterns ( constant intensity, duration, and timbre), based on the Western tonal system. Subjects compared reference patterns with comparison patterns, which were either identical to the reference pattern or nonidentical because of predetermined changes in the comparison patterns. The patterns were presented in auditory or visual mode in a slow or a fast version. The results show a striking cognitive response to nonidentical tones in the comparison patterns and to a lesser extent to exceptional tones even in the reference patterns. Different levels of response were detected according to the type of pattern. We also found some evidence of factors contributing to "subjective equivalence" between different patterns. The correlation between results from this study and those from earlier studies based on the same measuring technique with different kinds of input data or using different methods and techniques is discussed.