A simulation of the longitudinal nuclear-electromagnetic cascade development in an ionization spectrometer has been carried out using a Monte Carlo method. Details of the cascade model and the technique used for the calculations are presented. Calculations have been performed to correspond to an actual spectrometer which was exposed to 10-, 20.5-, and 28-GeV/c incident protons at the Brookhaven Alternating Gradient Synchrotron (AGS). Particle-number distributions predicted by the calculations for different depths in the spectrometer are compared with the measurements. Results predicted by the calculations at higher energies extending up to 1000 GeV are also given. Means and standard deviations of the energy leaking out of the bottom of the spectrometer, as well as the energy dissipated in the form of nuclear disintegrations within the spectrometer, are tabulated for various depths for primary energies ranging between 10 and 1000 GeV. The energy leaking out of the sides of the spectrometer during the AGS measurements is estimated. The calculations show that the sum of particles recorded by the spectrometer increases linearly with energy for events undergoing the first interaction in the top layer of the spectrometer. © 1969 The American Physical Society.