The dynamic tensile strenghts of four rocks have been determined. A flat plate impact experiment is used to generate ~1-ms-duration tensile stress pulses in rock samples by superposing rarefaction waves to induce fracture. A gabbroic anorthosite and a basalt were selected because they are the same rock types as occur on the lunar highlands and mare, respectivly. Although these have dynamic tensile strengths which lie within the ranges 153--174 MPa and 157--179 MPa, whereas Arkansas novaculite and Westerly granite exhibit dynamic tensil strengths of 67--88 MPa and 95--116 MPa, respectively, the effect of chemical weathering and other factors, which may affect application of the present results to the moon, have not been explicitly studied. The reported tensile strengths are based on a series of experiments on each rock where determination of incipient spallation is made by terminal microscopic examination. These data are generally consistent with previous determinations, at least one of which was for a significantly chemically altered (hydroxylated) but physically coherent rock. The tensile failure data do not bear a simple relation to compressive results and imply that any modeling involving rock fracture consider the tensile strength of igneous rocks under impulse loads distinct from the values for static tensile strength. Generally, the dynamic tensile strengths of nonpourour igneous rocks range from ~100 to 180 MPa, with the more basic, and even amphibole-bearing samples, yielding the higher values.