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The present study investigated the question of whether the previously observed impairments of working memory characteristic of dieting to lose weight can be explained in terms of preoccupying cognitions relating to body shape or to alterations in serotonergic function resulting from a low dietary intake of tryptophan. The population comprised female non-dieting, lower restrained eaters (N=23), non-dieting higher restrained eaters (N=11) and current dieters (N=19). Each participant completed three tasks, each of which selectively loaded on to a different sub-component of working memory. The tasks comprised the Tower of London task, a letter string recall task and a mental rotation task. In addition, all participants completed self-report measures of body shape concern and affective state. Serotonin turnover was assessed by means of 24 h urine sample collection for each participant on their day of testing. This was analysed (via HPLC) for levels of the main serotonin metabolite 5-HIAA.The results of the present study broadly replicated previous findings of a Central Executive and Phonological Loop (but not Visuo-Spatial Sketchpad) deficit in those subjects who reported themselves to be currently dieting. Tower of London task performance also significantly correlated with self-reported feelings of fatness and body shape disparagement. There were no group differences in 5-HIAA levels nor did 5-HIAA levels correlate with task performance. However, there was a significant negative correlation between 5-HIAA levels and self-reported depression. These results support the hypothesis that the variables mediating this deficit are preoccupying cognitions concerning body shape. They do not support the hypothesis that the serotonergic function of dieters is compromised, although this conclusion is tentative. [Copyright &y& Elsevier]