A New Method for Studying Rydberg Atoms Yanbang Wang, Florida International University, Miami, FL Argonne National Laboratory We apply two-photon resonant nondegenerate four-wave mixing with a resonant intermediate state for the observation of Rydberg states in sodium vapour. The broadening and shift of the sodium 3S-11D transition perturbed by argon are investigated. This technique can achieve Doppler-free resolution of narrow spectral structures of Rydberg levels if lasers with narrow bandwidths are employed.