Working memory (WM) usually refers to a cognitive system devoted to the simultaneous maintenance and processing of information which plays a crucial role in high-level cognition. Recently, Barrouillet and collaborators showed the importance of controlling the time course of cognitive activities to assess WM capacities. Therefore, they developed a new paradigm to systematically explore the functioning of WM that involved simple but time-constrained activities as processing component. In comparison with traditional tasks, these computer-paced span tasks provide a more accurate evaluation of WM capacities and turned out to be the most predictive of complex cognitive achievements. The present study was the first attempt to evaluate the improvement of working memory resulting from training by repetition of this type of span tasks. Participants were trained during twelve sessions with a span task in which the duration of the concurrent activity was varied to ease the implementation of an attentional refreshing mechanism. The transfer effects were evaluated with a similar span task with a different type of material to be memorized. Results showed a significant effect of training, but no transfer effect: trained participants did not outperform a control group, and their performance in the second task did not differ from the first task. Thus, we suggested that the improvement in recall performance does not rely on an increased efficiency of a domain-general process (i.e., refreshing), but on the discovery and use of more efficient encoding strategies.