The HORIZONTAL D-GASSER unit has performed reliably on hundreds of thousands of wells to remove virtually all entrained gases from drilling fluids. Gas-cut mud is drawn into the tank through a vacuum created by the discharge jet and the pump. The mud is dispersed in a thin layer over the full-length baffle and, as the entrained gas is freed, the mud is recovered by the vacuum pump. The gas is discharged at a specified safe distance from the drilling operation while the restored mud is returned to the drilling fluid system.