(57) A downhole drilling motor is described which has an outer drill casing (11), a shaft (12) for driving a drill bit connected thereto, and the shaft (12) being rotatable relative to the casing (11). A turbine (15) is disposed between the casing (11) and the shaft (12) to permit the shaft (12) to be rotated. A pressurised fluid can be admitted via an inlet (13) and is transmitted to the turbine (15) via a passage (20) to effect operation. The shaft is provided with reaction means which includes a sleeve (17) mounted on the shaft (12) and which sleeve and shaft have pressure surfaces (23, 24) which can be acted on by the pressure fluid to produce a force acting counter to the normal hydraulic thrust on the shaft (12) to produce a reduced resultant hydraulic thrust on the shaft (12). In a preferred arrangement, the reaction member and thrust bearing is located above the turbine (15) so that the unit can be replaced as a whole.