The traction in pure rolling has been measured at constant temperature over a range of load and speed. Measurements have been made at controlled temperatures of 10, 20 and 30 degrees C on a paraffinic and a naphthenic based mineral oil. The results form a systematic pattern and provide a fuller picture of the variation of the enhanced viscosity of the oil with load and rolling speed than has been available hitherto. It is shown that the viscosity tends towards a limiting value when the rolling speed is low. The results are compared with recent measurements by Hamilton & Moore of the variation of the pressure distribution with load and speed. Both sets of results appear to suggest that the response of viscosity to the application of pressure is not instantaneous, but is delayed with a characteristic relaxation time. However, quantitative examination shows that this hypothesis is untenable. It is then shown that the variation of viscosity with rolling speed originates from the variation of the pressure distribution with speed. In the region of highest pressure the response of viscosity to pressure is normal. In the inlet and outlet zones, the shear stresses are very high and in these regions the oils exhibit non-Newtonian behaviour.