We introduce a formulation of heavy-fermion systems under applied pressure, in which the cell-volume change resulting from the valence fluctuation between the f singlet and magnetic multiplet accounts for the pressure dependence of the energy gap in Kondo insulators. The variations with pressure of the low-temperature specific-heat coefficient and resistivity manifesting thermal-activated behavior are naturally derived. A comparison is made between the Yb-, Sm-, and Ce-based cubic Kondo insulators. \textcopyright{} 1996 The American Physical Society.