Ultra-fast Magnetic Resonance Imaging (MRI) is used to image liquid distribution during air-water, co-current downflow through a fixed bed of cylindrical porous pellets of length and diameter 3 mm, packed within a 43 mm internal diameter column in both the trickle- and pulsing-flow regimes. Two different data acquisition times are used, 20 ms and 6.4 s, giving two-dimensional images of in-plane spatial resolution 1.4 mm x 2.8 mm and 351 mum x 351 mum, respectively. Superficial gas velocities in the range 27-275 mm s(-1) (0.033-0.338 kg m(-2) s(-1)) and superficial liquid velocities in the range 0.4-13.3 mm s(-1) (0.4-13.3 kg m(-2) s(-1)) were used. This work reports images of local pulsing events within the bed occurring during the trickle-to-pulse flow transition. The evolution of the local instabilities is studied as a function of increasing and decreasing liquid velocity, at constant gas velocity. Temporal fluctuations in local liquid holdup were seen even in the trickle flow regime; these fluctuations had much shorter time periods than the pore-scale liquid pulses, and are perhaps associated with fluctuations in local surface wetting of the packing elements. A preliminary assessment of the effect of periodic operation on liquid distribution within the bed is also reported. (C) 2004 Elsevier Ltd. All rights reserved.