Room temperature friction experiments on quartzofeldspathic rocks obey a velocity dependence of strength which consists of two opposite-sensed effects. The second of these effects has a negative velocity dependence and evolves over a characteristic displacement. This evolution effect was originally attributed by Diettrich [1978; 1979> to an underlying time-dependent process but is often described by either of two empirical evolution laws. One depends explicilly on displacement (slip law) and the other retains time dependence (slowness law). The slip law is favored in representing behavior around steady-state as seen in velocity stepping experiments. However, in this study slide-hold-slide tests conducted at different machine stiffnesses show that the evolution effect depends on time, not slip. For the slowness law the coefficient of time-dependent strengthening b is measured directly in slide-hold-slide tests. Existing empirical evolution laws may not be sufficient to describe both near steady-state and non steady-state behavior Provided a more correct form can be found, time-dependent evolution may improve frictional models of the seismic cycle by reducing the amount of inter-seismic slip. ¿ American Geophysical Union 1994