The photographie effect of pressure was studied systematically. It is established that apart from the simple mechanical effect, which is caused by pressure alone,the mutual effect of pressure and exposure varies with the sequence of their application. X-rays were used throughout the investigation. If pressure is applied after exposure. the developability of the film is increased,the induction in density being proportional to the exposure. If pressure is applied before or during exposure, the developability may be either decreased or inereased,depending on the characteristics of the film used and the amount of the pressure applied. The effeet of pressure. when it is applied before exposure,is less pronounced than that in the ease of sinultancous application. The diference varies with t which is the time interval between pressure and exposure. If we plot the reduction in density D corresponding to a certain pressure and exposure against t, an equilateral hyperbola is obtained given by the equation （D-a)(t+β)=v. where a,β and y are constants.