Photoelectron spectroscopy was used to study the mode dependence of vibrational autoionization in high-Rydberg states of NH3. Two-color, two-photon resonant, three-photon excitation via the C' (1)A(1)'(1300) intermediate state was used to populate selected autoionizing Rydberg states between the (1200) and (1300) ionization thresholds of the NH3+ (X) over tilde (2)A(2)" ground electronic state, and the ionic vibrational distributions were determined from the photoelectron spectra. Excitation of Rydberg states in which two different vibrational modes are excited allowed the direct comparison of the autoionization efficiencies for the two modes. Autoionization via the loss of one quantum of vibrational energy from the nontotally symmetric "umbrella" mode, nu (2), was found to be the dominant autoionization process. Vibrational branching fractions obtained from the ionic vibrational distributions indicate that, for the Rydberg states accessed via the C-' (1)A(1)'(1300) intermediate state, the nu (2) mode is approximately 25 times more efficient at promoting autoionization than the totally symmetric stretching mode, nu (1).