Groups of brain damaged and normal children were subdivided into young (age range 6 to 9 years) and old (age range 10 to 13 years) groups and subjected to an auditory reaction time procedure consisting of regular and irregular preparatory interval conditions. The most important results were as follows: the performances of the young normal, old normal, and old brain damaged groups were superior to that of the young brain damaged group; there was no significant differences between the performance of the old normal and the old brain damaged groups; the set index formula was not more effective in differentiating between the performance of the young brain damaged and young normal groups or between the young brain damaged and old brain damaged groups than were other, more easily derived, indices. The results of the current investigation were highly similar to those obtained in previous studies which employed visual reaction time procedures (Czudner & Rourke, 1970, 1972). The results of all of these studies support the contention that, with advancing years, brain damaged children of the type used in the present study may adapt to and/or recover from the deficit(s) involved in the inability to develop and maintain a state of readiness to respond. (Author)