A torsional-impact machine has been developed to investigate the dynamic behavior of metals at large strains and high strain rates. Two machines are described; the first was used to continuously monitor stress, strain profile and temperature profile during deformation; the second was designed specifically to investigate the thermal-instability strain. In both machines, the use of a shear pin made independent control of the ultimate strain possible, permitting essentially constant strain rate throughout the test. Shear-strain rates of up to 2300/sec and ultimate shear strains of up to 400 percent were obtained on machine No. 1. Some experimental results are presented to indicate the type of data obtained and the error limits on measurements.