Event-related potential (ERP) studies of working memory have used delayed S1-S2 match-to-sample tasks in which S1 is held in memory for comparison with S2. ERP negativities in the S1-S2 interval have been interpreted either in terms of working memory operations, or in terms of general preparatory motor processing. Two experiments (N = 20 each) were carried out to explore the nature of ERP negativities in a visuospatial memory task and in an auditory spatial memory task, respectively. In the experimental condition, subjects had to memorize the location of S1 (S1-memorize) so as to respond whether S2 appeared in the same spatial location (S2-memorize). In the control condition, subjects were requested to ignore S1 (S1-passive), and to respond whether S2 matched or not a target location predetermined at the beginning of the trial block (S2-pressing). Results support the two main conclusions of Martin-Loeches et al. (Electroenceph. clin. Neurophysiol., 1994, 91: 363-373). Firstly, that the encoding into memory of spatial location is associated with an ERP negative wave over the brain areas putatively associated with the processing of sensory information (i.e. right parieto-occipital for the visual task; fronto-central and left temporal areas for the auditory task). Secondly, the P300 does not seem to be an important ERP feature related to spatial location encoding and retaining into memory. Despite the distinct scalp distribution of these memory-related, modality-specific ERP negativities, they also showed a considerable degree of temporal synchronicity across modalities.