A recognition RT paradigm was used to assess a number of plausible search strategies in LTM for categorized lists. List length was determined by factorially combining two, three, or five categories with two, three, or five words per category, and test items could be one of four types: (1) positive, (2) a repeated positive, (3) a related negative, or (4) an unrelated negative. It was found that RT increased linearly with category size for both positive and related negatives (about 30-40 msec/item); whereas the increase was much smaller for the unrelated negatives, especially for three and five categories (about 9 rnsec/item). With an increase in the number of categories, RT increased at the rate of about 40 msec/category for all three test items. A theory of high-speed scanning for categorized material was proposed in which a serial and exhaustive search-of the categories is rust undertaken. If a positive category match is found, a serial and exhaustive search within the contents of the positive category is initiated; if no category match is found, the search is simply terminated. Some evidence was presented that categories recently probed may provide for a short-circuiting of the category search.Over 100 years ago, Donders (1868) suggested a method for studying the characteristics underlying simple mental processes, e.g., time required to detect a stimulus, time required to discriminate between two or more stimuli, etc. Donders's method contained two main assumptions: (1) Mental events require "real" time, i.e., their execution is neither instantaneous nor is the event itself too brief for measurement, and (2) complex mental operations are composed of simpler substages which occur serially and whose time components are additive. After some initially promising results, the second assumption was criticized by Kulpe (1896), leading to a diminution of interest in the Donderian additive model.Recently, however, the additive model was revitalized by Sternberg (1966Sternberg ( , 1969 in his study of retrieval processes from short-term memory (STM). Ss were required to make one response (e.g., pull a lever) whenever a stimulus from a predefined set was presented and to make a different response otherwise (e.g., pull a different lever). The size of the positive set was varied from one to six items on any particular trial. Sternberg interpreted the results as indicating that retrieval of information from STM was serial and exhaustive; the search was serial because reaction time (RT) increased linearly with the size of the positive set and exhaustive because of the equivalence of the slopes for the positive and negative stimuli. From the slope constant, it was hypothesized that each item of the positive set was successively compared to the test stimulus at a rate of about 40 msec per item.