The rotational dynamics of water in super- and subcritical conditions is investigated by measuring the spin-lattice relaxation time T-1 of heavy water (D2O). The experimentally determined T-1 is shown to be governed by the quadrupolar relaxation mechanism even in the supercritical conditions and to provide the second-order reorientational correlation time tau (2R) of the O-D axis of a single water molecule. It is then found that while tau (2R) decreases rapidly with the temperature on the liquid branch of the saturation curve, it remains on the order of several tens of femtoseconds when the density is varied up to twice the critical at a fixed supercritical temperature of 400 degreesC. The comparison of tau (2R) with the angular momentum correlation time shows that the rotational dynamics is not diffusive in supercritical water. The dependence of tau (2R) on the hydrogen bonding state is also examined in combination with molecular dynamics simulations, and the effect of the hydrogen bonding on the rotational dynamics in supercritical water is found to be weaker than but to be on the same order of magnitude as that in ambient water on the relative scale. Actually, although tau (2R) is divergent in the limit of zero density, it is observed to increase with the density when the density is above similar to1/3 of the critical. (C) 2001 American Institute of Physics.