2S-O aluminum was extruded at room temperature in an inverted extrusion process from a 4.3-in-diam cylindrical billet, into a 1.5-in-diam solid cylindrical bar. Steady-state particle-velocity vectors were determined from gridded, split billets during incremental extrusion steps. These velocity patterns were compared with those obtained previously for lead extrusions having the same billet and die geometry and the same high degree of lubrication; it was found that the patterns for these two metals were identical. The aluminum velocity pattern appeared not to be affected by extrusion load, but the extrusion load was a strong function of the extent of deformation. It was estimated that the extrusion load becomes constant after a reduction of billet length by approximately 2.4 in., the distance the plastic zone of the metal extends upstream from the orifice. The axial-stress distribution in the billet, determined front strain rates and natural strains, taking into account work hardening, had the same shape as that determined with lead. The wall pressure or radial stress in the plastic zone was found to be lower than the average pressure, but both pressures tend to approach each other beyond the plastic zone.