This work has consisted in demonstrating that high gain can be achieved by pumping x-ray lasers (XRL) with a combination of a high intensity and short duration driving pulses (approximately 100 ps). Short pulses are very well suited for pumping collisional XRL since a high lasant ion density, electron density and temperature can be achieved simultaneously. We have successfully tested this pumping scheme on the 4d-4p (J equals 0 - 1) transition of Ni-like tin (lambda approximately 11.93 nm) and silver (lambda approximately 13.89 nm) as well as on the 3p-3s (J equals 0 - 1) Ne-like iron (lambda approximately 25.5 nm) at an intensity of approximately 2 X 1013 Wcm-2 (130 ps in duration). The driving laser (lambda equals 1.06 micrometer) was composed of three pulses (a prepulse and two main pulses). Large amplifications were demonstrated in tin and silver (respectively GL approximately 12 and GL approximately 16). Finally, the saturation of the 3p-3s (J equals 0 - 1) transition of Ne-like iron at 25.5 nm was achieved on both pumping pulses, using a prepulse of 109 Wcm-2. A gain coefficient of 15 plus or minus 3 cm-1 (GL approximately 26 plus or minus 5) on the first main pulse and 12 plus or minus cm-1 (GL approximately 23 plus or minus 2) on the second one was measured.