An experimental procedure is described for accurately establishing the dynamic fracture initiation properties of structural metals at extremely high loading rates. The apparatus is an adaptation of the Kolsky pressure bar (split-Hopkinson bar) in which a 1-in.-diameter round bar specimen with a prefatigued circumferential notch is loaded to failure by the rapidly rising tensile pulse resulting from an explosive detonation. Using the standard Kolsky technique, the average stress at the fracture site is measured as a function of time. Crack opening displacement is measured by optical means, as a function of time, thus yielding a complete load-displacement record for each test. From the data the critical value of the crack-tip stress intensity factor, K_1c, at loading rates, ˙K₁ in excess of 10⁹ psi √in./s may be obtained. This is nearly two orders of magnitude faster than has been achieved by other standard techniques. Results are presented for dynamic tests conducted on SAE 4340 steel and 1020 cold-rolled steel, and these are compared to results from static tests performed on specimens of similar shape.