Transient natural convection in a low-Prandtl-number fluid was computed for a shallow rectangular enclosure of 5 x 5 x 1 heated from below and cooled from above with four dragless, adiabatic, vertical walls. The model consists of three-directional mass, momentum and energy balances. These equations were approximated by a HSMAC finite-difference method with a hybrid scheme for the inertial terms. Computations for Ra = 2000 and Pr = 0.01 required an accumulative time of two months. The transient response after a step increase of the lower-wall temperature was at first a regular oscillation, which then evolved to a series of long rolls with their axes in one direction. These transient characteristics of the velocity and temperature fields are presented graphically.