We have measured the pressure dependence of the Knight shift (K) for 141Pr in PrP and 169Tm in TmP at 4 K and find d ln K/dP=0.83% and 0.48%/kbar, respectively. Both compounds are paramagnetic but exhibit a temperature independent (Van Vleck) susceptibility near T=O due to the crystal field induced singlet electronic ground state. The rare earth Knight shift provides a direct measurement of the susceptibility (Xvv), from which the crystal field splitting (Δ) may be determined, i.e., Xvv∞ Δ−1. Thus, our result of d ln K/dP≳O is in qualitative disagreement with the usually successful effective point charge model. Such a model implies that Δ varies as some inverse power of the lattice constant and, consequently, that the pressure derivative (d ln K/dP) is negative. We checked for anomalous conduction electron‐nuclear coupling effects by measuring the pressure dependence of the 31P Knight shift in PrP. However, the observed d ln K/dP (1.6% kbar) excludes this possibility and probably results from the same susceptibility change that is responsible for the rare earth resonance behavior.