A method of dilatometry using x-ray illumination has been developed and applied to measurements of compressibility. Metal samples, 10 in. long and 316 in. in diameter, are subjected to pressures up to 10,000 atmospheres. Observations of the length of the compressed sample are made by causing x-rays to cast a shadow on a photographic plate. The changes in length of the shadow are then determined with a comparator. This technique, which has been used to measure the compressibilities of aluminum, iron, and copper, is easily adaptable to remote measurements of any type involving changes of length, e.g., thermal expansion.