5 groups of Ss trained with ISIs of 0, 0.5, 1, 2, and 5 sec. were used in a study of trace alpha blocking “conditioning.” CS was a 0.5-sec. tone, and US was a single light flash. 20 female college students showing at least 50%-time alpha rhythm in their resting EEGs served as Ss. Alpha blocking Rs were defined as a 25% reduction in alpha amplitude during the 0.5 sec. of the CS (initial R) and the 0.5 sec. immediately preceding the US (trace R), as compared to the 0.5 sec. immediately preceding the CS. Analysis of variance of the Rs showed no effect from ISI, a highly significant response effect, with trace Rs more numerous than initial Rs, a highly significant trials effect, and a significant response × trials interaction. Trend analysis of the trials and the interaction confirmed and extended these results by showing significant over-all linear, cubic and quintic components. Initial Rs consistently decrease (habituation), while trace Rs showed significant decreases and increases. Initial Rs were attributed to sensitization, while two types of pre-US or trace Rs were identified: one, at the beginning of “conditioning”, possibly related to initial Rs and undergoing fast habituation, and “true” trace Rs which developed after 20 trials and subsequently habituated slower than initial Rs. Differences between trace and delayed alpha blocking Rs were also discussed.