The movement of lime-and lithium-base greases in a transparent model roller-bearing has been studied. Within the first few revolutions air pockets are formed between the regions of grease moving with different velocities in various parts of the bearing, and the grease in these regions is observed to be undergoing differential amounts of mechanical working. Electron micrographs of grease taken from selected regions of bearings run with lime-and lithium-based greases show that the fibre structure of grease taken from the rolling surfaces is almost completely destroyed whereas the grease taken from the relatively immobile grease on the cage undergoes very slight change. These structures are compared with those of greases subjected to heat and to conventional mechanical worker tests.