Sternberg paradigm is one of the neuropsychological tests that assesses working memory. In this study two Sternberg experiments with letters or meaningless figures were used in order to determine the electrophysiological correlates of distinct subsystems of working memory. In each block, 3 and 5-item memory sets were presented with 2 s intervals to the subjects on the computer screen. After 3 s, a set of 6 or 10 probes were presented, respectively, and the subjects were told to push the left button of the computer mouse if the subject thought that the stimulus on the screen belongs to the memory set, and the right button of the mouse if not. Eighteen right-handed, healthy volunteers with a mean age of 25.9 took part in the study. 30-channel-ERP recordings were carried out during the tests. Mean amplitudes of the ERP waveforms were measured for 15 intervals of 50 ms duration between 0 and 750 ms. The analysis of the reaction times (RT) of 3 and 5 item memory sets shows that each additional item in the memory set increases the RT by 28 ms, which is in accordance with the earlier studies. In ERPs, the most important finding was a right lateralized positivity consistently observed at 200 ms after stimulus presentation in each condition, which seems to reflect the beginning of the comparison of the presented stimulus with the memory set. Following this, a P300-like potential with a midline parietal maximum was observed whose latency was prolonged in line with the RTs with increasing memory load. Distractors and meaningless figures further prolonged the latency of this late potential, which seems to reflect the end of the comparison and the decision making process.