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Abstract This letter compares the predictions of two expressions proposed for the porosity evolution in the context of rate and state friction. One (Segall & Rice, 1995,https://doi.org/10.1029/95jb02403) depends only on the sliding velocity; the other (Sleep, 1995,https://doi.org/10.1029/94jb03340) depends only on the state variable. Simulations of both are similar for velocity stepping and slide‐hold‐slide experiments. They differ significantly for normal effective stress jumps at constant sliding velocity. Segall and Rice (1995,https://doi.org/10.1029/95jb02403) predicts no change in the porosity; Sleep (1995,https://doi.org/10.1029/94jb03340) does. Simulation with a spring‐block model indicates that the magnitude of rapid slip events is essentially the same for the two formulations. Variations of porosity and induced pore pressure near rapid slip events are similar and consistent with experimental observations. Predicted porosity variations during slow slip intervals and the time at which rapid slip events occur are significantly different. The simulation indicates that changes in friction stress due to pore pressure changes exceed those due to rate and state effects.