Plane shock wave experiments have been conducted on two highly porous rocks, Mount Helen tuff and Indiana limestone, in both dry and water-saturated states up to stress levels of about 4 GPa. A light-gas gun was used to load the sample in uniaxial strain, and subsequent wave motion was monitored with particle-velocity gages. All four materials studied show evidence of time-dependent behavior. The time-dependent behavior in the highly porous dry rocks is associated with the closing of pores. The strong time dependence observed in these materials would seem to preclude the use of quasi-static data in constitutive models that are used to describe dynamic processes. In the water-saturated rocks the time dependence is associated with the water, which shows no indication of transformation to the high-pressure ice phases in the time frame of these shock wave experiments. This suggests the possibility of a metastable form of water existing under dynamic conditions.