A high speed stress‐strain machine has been developed which is capable of recording the stress‐strain curve of elastomers at elongation rates up to 270 percent/msec. Data are reported on two series of gum and tread stocks of hevea and of the synthetic elastomers, GR‐S, Hycar OR, butyl, Perbunan, and Neoprene GN. The second (elastomer) series was also run at 150°C. In general, stress‐strain curves fall into two classes. Stocks of elastomers which are known to crystallize on stretching tend to show tensiles which decrease with increasing speed up to about 10 percent/msec., pass through a minimum and rise more or less drastically to values 100 percent (or more) greater than the Scott tensile. Elastomers which do not crystallize on stretching tend to show a steady rise in tensile with increasing speed. Elongation at break curves show a maximum with crystallizing stocks and no maximum with non‐crystallizing stocks. The shape of the modulusversusspeed curves is accounted for on the hypothesis of different types of slipping bonds with different characteristic relaxation times. The shift of curves for tread stocks with temperature allows the estimation of a heat of activation of slippage. This comes out to be of the order of 3 kg cal.