An improved method for high-electron-density (10(17) cm(-3)) measurement in a shock-generated plasma is described. A laser interferometer operated at 3.39 mum is regularly modulated (1 MHz) by a LiNbO(3) electro-optical crystal. Thus electron-density variations of 8x10(15) cm(-3) mus(-1) and fluctuation of 8x10(14) cm(-3) can be simultaneously and accurately measured in a 4x4-cm conventional shock tube. Experimental results in reflected shock waves are presented.