The ultrafast optical Kerr response of water is explained by a theoretical model of the liquid dynamics, which evidently yields the ultrashort lifetime, of approximately 120 fs, of the momentary tetrahedral short-range order among the nearest neighbours. The slowly decaying component of the response is shown to be due to, both, the rotational diffusion of non-hydrogen bonded molecules and the overdamped translational oscillations of hydrogen-bonded molecules. The numerical calculations performed both for water and heavy water show a very good agreement with the experimental data. (c) 2006 Elsevier B.V. All rights reserved.