The development of hydraulic transmissions has resulted in an increased consumption of hydraulic fluids. In the French Coal Mines, mineral oil which still remains the most effective hydraulic fluid is still applied underground because there is no mandatory regulation prohibiting its use. Mine operators have substitued fire-resistant hydraulic fluids in those applications where ignition hazards are greatest; such fluids were introduced underground after being subjected to tests and controls based on precise recommendations of the Permanent Committee for Safety and Health in the Coal Mining Industry.Because of their simplicity of design, hydraulic couplings were adapted to use of non aqueous hydraulic fluids. Powered supports and some unsophisticated equipment are operated with soluble oil in water emulsions. Most equipment have been converted for using poly-glycol solutions. The equipment still using mineral oil will either gradually be replaced by new equipment specially designed to operate with fire resistant fluids or modified accordingly. The final objective is to increase safety conditions underground while permitting normal productivity of operation.