Abstract: A regulator for underwater diving having an improved first stage wherein the bias of the coil spring is varied by changing the relative position of the piston seat which can be readily accessed. The second stage of the regulator is capable of satisfying the human breathing demand with the use of a linkage ratio adjustment feature which coupled with the selection of the proper angle for the housing of the diaphragm permits the amplification and faithful tracking of the diver's breathing cycle. Also, exhaust means has been provided to facilitate the purging of extraneous fluids from the regulator in the most common positions of the diver. The above mentioned features, in combination, will provide a perfect match of the first and second stages of the regulator.