We report first investigations of high principal quantum number Rydberg states of the neutral beam photoionization spectrometer recently developed at Freiburg. A fast beam of metastable D-3 molecules was created by charge transfer of a D-3(+) beam in cesium. Rydberg states of D-3 were analyzed by pulsed-laser excitation using two-photon resonance-enhanced ionization, electric field ionization and autoionization. Our data identify the 2p (2)A(2) " = (N = K = 0) state of D-3 to be metastable with a lifetime of about 1 mu s. The spectral lines of following excitation in the ultraviolet spectral range were assigned to s-type and d-type Rydberg-series converging to vibrational ground state, symmetric stretch excited and degenerate mode excited D-3(+) ion states. By a contribution of vibrationally diagonal and non-diagonal transitions, we determined the ionization potential, the symmetric stretch and degenerate mode vibrational frequencies of the 2p 2A(2) " state in D-3. The data give insight into the influence of the coupling between the Rydberg electron and the ion core on the potential energy surface.