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This paper reconsiders the notion of slack bus in load-flow studies. Instead of determining a priori which bus plays the role of slack bus, it is selected on the fly during the load-flow iterative process in such a way that the system power imbalance is minimized. The problem of selecting the best slack bus, or buses, is first formulated as a nonlinear optimization problem. Then, the results obtained are justified on the basis of the involved equality constraint being a quasilinear function, leading to an LP problem with trivial solution. It turns out that the optimal solution can be easily found from the results of a conventional load flow at a moderate cost. The proposed heuristic procedure is tested on the IEEE test systems.