We present the results of theoretically simulating, by variational methods, rotationvibration spectra of NH3 and PH3. The simulations carried out for NH3 are based solely on ab initio calculations, i.e., they are purely theoretical and involve no fitting to experiment. The PH3 simulations are made from a potential energy function refined to reproduce experimental data and from an ab initio dipole moment function. We show that our simulations reproduce observed rotation-vibration intensities with an accuracy approaching that obtained in fittings to these intensities in terms of models involving an effective dipole moment operator. Our results suggest that theoretical simulations of spectra are now close to attaining a level of accuracy where they can successfully compete with quantitative-spectroscopy measurements of intensities and thus assist in the interpretation of remote-sensing spectra.