An undergraduate exercise in computational chemistry that investigates the energy barrier for pyramidal inversion of amines and phosphines is presented. Semiempirical calculations (PM3) of the ground-state and transition-state energies for NR[superscript 1]R[superscript 2]R[superscript 3] and PR[superscript 1]R[superscript 2]R[superscript 3] allow for the determination of the activation energy for pyramidal inversion. Various factors affecting the inversion activation energy are considered. The exercise can be done easily in a 3-h lab on a standard laptop running Spartan or Spartan Student or as a homework assignment. (Contains 2 figures and 7 tables.)