Dynamic Young's moduli are derived from observations of wave propagation in stretched filaments of natural and butyl rubbers. Extensions up to about 700% are studied within a temperature range of −45 to +50°C. and a frequency range of 0.2 to 20 kycles/sec. At constant temperature and frequency both the real (E') and imaginary (E″) components increase with increasing extension. The loss factor (E″/E') decreases with increasing extension, but the decrease is more pronounced for the butyl rubber so that the large difference in damping between the two polymers in the unstrained state is greatly reduced at high extensions. Approximate relaxation spectra are derived for the strained materials on the basis of several assumptions regarding the applicability of the WLF equation. It is found that the spectra are displaced in the direction of longer times by increasing extension; this is equivalent to an increase in the monomeric friction coefficient with extension. The effect is more pronounced for the natural rubber than for the butyl, and consequently the ratio of the friction coefficients (butyl:natural) falls from several hundred unstrained state to approximately 3 at 550% extension.