Working memory is mediated by the discharges of neurons in a distributed network of brain areas. Recently, it has been suggested that enhanced rhythmicity in neuronal activity may be critical for sustaining remembered information. In order to test whether working memory is characterized by unique temporal discharge patterns, we analyzed the autocorrelograms and power spectra of spike trains recorded from the posterior parietal cortex of monkeys performing a visuospatial working memory task. We compared the intervals of active memory maintenance and fixation and repeated the same analysis in spike trains from monkeys never trained to perform any kind of memory task. The most salient effect we observed was a decrease of power in the 5-10 Hz frequency range during the presentation of visual stimuli. This pattern was observed both in the working-memory condition and the control condition, although it was more prominent in the former, where it persisted after cue presentation when the monkeys actively remembered the stimulus' spatial location. Low-frequency power suppression resulted from relative refractory periods, which were significantly longer in the working-memory condition and presumably emerged from local-circuit inhibition. We also detected a spectral peak in the 15-20 Hz range, however this was more prominent during fixation than during the stimulus and working-memory periods. Our results are in line with previous reports in prefrontal cortex, and indicate that unique temporal patterns of single-neuron firing characterize persistent delay activity, however these do not involve the appearance of enhanced oscillations.