The acoustic vibrational modes of Au nanorods with aspect ratios between 2 and 5 have been investigated by time-resolved laser spectroscopy. The results show that laser excitation launches a coherent vibrational motion that has a period that depends linearly on the length of the rod. Due to polydispersity in the samples, the measured period also depends on the probe wavelength (i.e., different probe wavelengths interrogate different length rods). Analysis of the data provides information about the size distribution of the sample and the homogeneous line width of the longitudinal plasmon band of the rods.