Thick gouge is extruded from between sample contacts of simulated fault zones. Thin gouge is locally extruded by irregularities of the fault surfaces. Extrusion causes the simple shear strain rate ϵ˙ to vary as a function of position within the gouge and with time. The local shear traction thus varies from its macroscopic value. The case of thick gouge is quantitatively modeled. Extrusion does not significantly affect experiments measuring a and b in the traditional formula for the instantaneous coefficient of friction μ=μ0+a ln(ϵ˙/ϵ˙0)+b ln(&psgr;), where μ0 is the steady state coefficient of friction at the reference strain rate ϵ˙0 and the state variable &psgr; depends on the previous history of the sample. Other effects are modest and most likely to be observable when a>b, implying that the strain rate does not spontaneously localize. The macroscopic coefficient of friction for a>b is decreased by an amount which is less than the short/long aspect ratio of the gouge zone. Measurements of frictional dilatancy in velocity-stepping experiments and of the effects of sudden changes of normal traction on shear traction are not compromised if the plates bounding the fault gouge behave as rigid parallel planes. The changes in shear traction after a sudden change in normal traction are strongly affected if the walls confining the gouge are compliant and do not remain parallel. Then the local ratio of the new normal traction to the old normal traction varies with position on the fault surface. ¿ 1999 American Geophysical Union