During a round trip, drilling mud is aged under well bore conditions, ie. high temperature and pressure. This is able to be utilized to evaluate effect of temperature and pressure on mud and its additives. A field test of mud is carried out according to this concept on September, 1976, at Shinsekihara well. Mud properties are measured continuously every twenty minutes for three hours after a round trip at well depth of 2, 289m, and, also, five mud samples are taken for laboratory work. Following conclusions are obtained. 1) A break of mud circulation, during a round trip can be utilized to evaluate effect of temperature and pressure on mud and its additives. 2) On the field test, apparent viscosity increases with circulation time and falls to a stable level after one cycle of mud circulation. This phenomenon is analyzed to clarify effect of aging. 3) Yield value of mud suffers effect of aging more than plastic viscosity in this test. It is assumed that a dispersing agent in the mud is decomposed during aging under the well bore conditions. 4) With comparison between mud properties measured at the well site and that measured at the laboratory, effect of temperature on mud is clarified. Plastic viscosity of mud decreses with increase of temperature, but yield value increses with increase of temperature. 5) Apparent viscosity of mud behaves complicatedly with temperature, so it is not suited as an index of mud property.