It is important for future ultrastructural preservation studies to freeze biological specimens as rapidly as possible. Finite element numerical techniques have been used to compare different cooling block materials used in the quick freeze 'slamming' method. It is concluded that a pure silver block at an optimum initial temperature of about 15.6 K will produce the quickest cooling and will give rates approximately 50% higher than if the block were initially cooled to 4.2 K. A copper block will produce the fastest cooling rates when the block is initially at liquid nitrogen temperature of 77.3 K provided the section being studied is taken no more than about 30 microns from the specimen-block interface. Composite metal blocks can further reduce the total specimen cooling time compared to a mono-block at the same initial temperature but the cooling rate near the interface will be slower. Future methods for specifying cooling rates within thin biological samples should be standardized and must be based on a combination of finite element numerical analysis and the measured temperature at the rear surface of the sample. Thermal contact resistance between cooling block and specimen and between specimen and the rear face temperature sensor must be minimal.