[en] Among the most important motivations for recent interest in medium- and high-energy heavy-ion reactions are the probing of the equation of state of nuclear matter, the possible study of nuclear systems at high densities and temperatures and their manifestations in exotic phenomena such as pion condensation and shape isomers. For many such investigations, nuclear hydrodynamics appears to provide an appropriate framework. However, for the validity of nuclear hydrodynamics it is crucial that the interaction time be long compared with the time necessary to bring the system into local equilibrium. Head-on collisions involving large nuclei present the most favorable cases where nuclear hydrodynamics may provide a valid description for the reaction process. Accordingly, a critical test of nuclear hydrodynamics will be the confrontation of theoretical predictions and experiments for head-on and near-head-on collisions