Experiments were carried out in a specially designed 5 m tall, 0.025 m i.d. high-density gas-solids downflow fluidized bed to measure the axial pressure gradient profiles along the downer and the actual solids holdup in the fully developed region. Fluidized catalytic cracking (FCC) particles ( rho (p) = 1300 kg/m(3), d(p) = 70 mum) and two types of glass beads ( rho (p) = 2500 kg/m(3), d(p) = 123 and 332 mum) were used. A particle acceleration region and a fully developed region were identified along the column from the pressure gradient profiles. In the fully developed region, solids holdup decreases with the gas velocity but increases linearly with the solids circulation rate. The latter results in nearly constant particle velocities over a large range of solids flux at given gas velocities. Particle velocity also increases linearly with the gas velocity. Particle properties seem not to affect the mean particle velocity much, but smaller and/or lighter particles give larger solid holdups. A solids holdup as high as 10-20% has been achieved. Comparison of the results obtained here with those from an upflow riser shows inherent similarities between the two gas-solids co-current flow systems. In the fully developed region, the apparent solids holdup calculated from the pressure gradient agreed well with the actual solids holdup measured by a pair of pinch valves under not very high gas velocities, but was underestimated at higher gas velocities due to the increased wall friction loss.