Many reinforced concrete structures utilize flat plates to provide lateral resistance when architectural constraints prevent wide-spread use of shear-walls. Understanding the resistance of flat - plate frames to large lateral loads is important for serviceability as well as seismic vulnerability assessment of hundreds of buildings on the East Coast. In February of 1991, the authors collected ambient vibration measurements to study the behavior of four flat plate, reinforced concrete structures in Manhattan ranging from 27 to 52 stories. All but one structure rests on a rock supported foundation. This report presents the development of ambient vibration analysis software based on the fast Fourier transform. Long ambient vibration records allow accurate power spectrum estimation. A robust peak picking method, also tailored for ambient vibration data, facilitates the interpretation of autopower and phase spectra. Damping estimates based on spectral peak band- widths are very sensitive to bias and leakage errors in the spectrum estimate. However, root-mean-square statistics of acceleration as well as of velocity and displacements can reasonably be estimated from the auto-power spectrum of response acceleration. Measured fundamental periods are shorter than those calculated from the 1982 UBC formula, but are longer than those calculated from the 1988 UBC Code or the proposed NYC Seismic Code. The results presented herein provide a base-line set of periods, deflections, and damping ratios to be compared to results expected to be obtained during strong winds. Periods estimated from ambient, mostly windinduced vibration measurements are used to calibrate finite element models of the structures, and are compared to values calculated from code design rules. A future goal of this on-going research is to relate effective flat plate width to lateral load level at service load levels, i.e., between ambient conditions and strong winds. Estimating dynamic parameters under various loading conditions coupled with ultimate load tests on model structures is intended to reveal the load-dependent stiffness of these structures (AU)