Visual search data were collected from six Ss on three target set sizes on each of 30 days. Error level was low, and items assigned to memory sets were nonnested and changed from session to session. For each S. the same item sometimes required a positive and sometimes a negative response (response inconsistency). Combining data over Ss and over successive 6-day blocks. visual search rates as a function of target set size were found to be linear for each of the five 6-day blocks. The slopes of the above functions (memory search time) did not differ significantly over the tmal four 6-day blocks. and averaged approximately .500 sec per six-character item. These results are qualitatively very similar to results obtained from item recognition studies when error level, memory set structure. degree of response consistency. and practice are handled in the same way in that task. The significantly lower slope obtained on the first 6-day block is shown to be consistent with a speed-accuracy tradeoff interpretation when error rate is expressed per unit of processing time (percent errors/set size). Over the fmal three 6-day blocks. where all important parameters of the data were highly stable. the intercepts of the memory search functions were found to closely approximate zero, averaging .0068 sec. From this finding, along with the finding that the memory search functions are linear, it is inferred that visual search time is determined entirely by memory search time, or by memory search time and other processes which increase linearly with set size, under the conditions of this experiment. The estimate of memory search time (approximately 83 msec/character) obtained using this visual search procedure is much slower than that obtained using the item recognition procedure (approximately 35-40 msec/character). An explanation for this difference is proposed.From a series of previous studies (Kristofferson, 1972a, b, c), it appears that the variables sufficient to control and predict the effect of the number of items in the memory set (the positive set size or the target set size) on item recognition time and visual search time are: (1) error level, (2) positive set structure, (3) degree of response consistency, and (4) practice.In the item recognition task, the S must respond "yes" or "no," by pressing the appropriate one of two keys, to indicate whether a presented test item is or is not a member of a predefmed positive set of items. The items are usually digits or letters. The size of the positive set is varied, and response latencies provide the basic data. Item recognition time (response latency) increases as positive set size increases (the item recognition function). The function is linear and has the same slope for both positive and negative responses. Typically, item recognition time is increased by approximately 35-40 msec for each additional item assigned to the positive set. This description of the item recognition function is accurate only for a particular combination of the variables listed above: (I) error level i...