Two-pulse photon echo electron-nuclear double resonance (or PENDOR) has been used to measure the second-order hyperfine splittings in the ground state and the lowest component of the excited D21 state of YAlO3:Pr3+. The excited-state splittings are 0.923 and 1.565 MHz and are significantly different from previous hole-burning results because of greatly increased resolution. The mechanism for two-pulse PENDOR involves a modulation of the photon echo decay at the rf Rabi frequency. This modulation has been observed directly. A brief theoretical analysis is given which shows that the echo modulation is a result of phase and amplitude changes in the oscillating opitical dipole moment which are indirectly produced by the rf field. © 1982 The American Physical Society.