The dynamic compression behaviour of the wet fibre mat is the key factor to understanding the mechanisms of wet pressing. At present this behaviouris imperfectly understood. The main reason for this may be the lack of suitable measuring equipment. To fill this gap, laboratory devices (press simulators) were built at the Finnish Pulp and Paper Research Institute. The simulators permit controlled pressing to be performed within a wide speed range, the fastest pressure rise being equivalent to that encountered in the presses of the fastest paper machines. The web thickness and the hydraulic pressure created in the web during pressing can be measured acturately. Papers with grammages of 50 and 100 g/m2 and with various furnish compositions were tested with these devices.