Laboratory data indicate that fresh water and filtrates from fresh-water base muds significantly reduce the effective oil permeabilities of water-sensitive cores; whereas various salt solutions and filtrates from saline muds cause little reduction and, in some cases, produce an increase. A simple test is described for evaluating the probable effect of mud filtrates on the effective oil permeabilities of water-sensitive cores. The mechanism of the interaction of mud filtrates with intergranular clays is discussed and several novel experimental observations are presented. The effects of mud particles on inert alundumcores have been studied by tests in a new type of mudding-off cell in which mud and bit action can be simulated under temperature and pressure conditions approximating those in the well bore. A few addtional static tests have been performed at high pressure and high temperature on natural cores from the Gatchell zone of the northeast Coalinga Field. Datafrom the dynamic mudding-off tests with oil-base, clay-water base, and emulasion muds indicate that mud particles penetrate through the inert cores and that filtrate losses for the particular cores used are an inverse function of their permeabilities. A hypothesis, based on the observed mud-particle invasion, is presented which appears to explain the apparent dependence of filtrate losses on rock permeability. Mudded-off cores have shown initial lossesup to 100 percent in the effective oil permeability, but subsequent deplastering tests have indicated in general that little permanent impairment of the effective oil permeability of inert cores is caused by mud-particle invasion. However, the deplastering data from tests performed under jetting-scraping conditions indicate that up to 35 percent permanent reduction in effective oil permeability may occur in the cores of high permeability. It is concluded that it is more desirable to complete wells with oil-base drilling fluid than with clay-water base or emulsion drilling fluids because of the extremely rapid recovery of oil permeability for cores mudded off with the former compared to the recoveries for cores mudded off with either of the latter. In general, it appears that there is greater impairment of permeability produced from the interaction of fresh-water filtrates with water-sensitive cores than from the invasion of mud particles into inert cores.