Immediate recall of word lists showed significant impairment after one night of sleep loss. Since S was required to write down each word immediately after its presentation, the deficit was not due to failure of sensory registration. With 24-hr, delayed testing, a picmre-recognition test did not show significant deficit after one night of sleep loss. Performance on this test was impaired, however, after a night of recovery sleep. These results imply that moderate sleep loss causes deficit in formation of the memory trace rather than in storage or retrieval functions and that this effect is probably independent of the physiological lapses (brief periods of sleep) which affect vigilance and sensory registration.As Weiner ( 1966) pointed out, most theorists see memory as consisting of a sequence of processes such as sensory registration, memory-trace formation, trace storage and retrieval. An important problem for the investigator of memory deficit is to specify which of these functions is impaired by a given experimental treatment. For example, memory deficit found with sleep deprivation (Patrick & Gilbert, 1896; Weiskotten & Ferguson, 1930;Edwards, 1941) could result from impairment of any or all of the functions mentioned by Weiner.Gieseking, Williams, and Lubin (1957) found that sleep deprivation caused progressive deficit in retention of general information items. Immediate recall scores deteriorated significantly after one night of sleep loss, with increasing decrement as sleep loss increased. Relative to immediate recall, delayed recall (24 hr. after initial presentation) showed no significant sleep-loss decrement up to 50 hr. of wakefulness. Williams, Lubin, and Goodnow (1959) attributed the deterioration of immediate memory to the occurrence of lapses (periods of sleep lasting 3 or 4 sec.) which were known to increase in frequency and duration as sleep loss increased. If a lapse occurred during the presentation of an information item, S might fail to register it for later recall. An adequate test of this explanation requires that sensory registration be brought under control. The experiments reported here attempted to confirm the deleterious effects of acute sleep loss on memory previously reported and to indicate which memory functions were most sensitive t o sleep loss. There are cwo experiments. The first tests whether sleep loss leads to impairment of immediate recall under con- 'We wish to thank Mrs. Elizabeth Engle, Mrs. Ometta Kearney, and Miss Betty Shanks for their assistance in test administration and statistical analysis.