To determine the effects of pressure on ionic conduction mechanisms the a.c. conductivity of AgPO3 and (Ag2S)(0.3)(AgPO3)(0.7) superionic glasses has been measured under hydrostatic pressure at selected temperatures, in the radio-frequency region. The experimental data show a monotonic decrease in conductivity at increasing pressure up to 2 kbar, while the onset of the dispersive a.c. conductivity depends on the pressure at fixed temperature. The data have been analyzed and compared with the temperature dependence of the a.c. conductivity obtained previously.