The formation of pressure-induced casein structures (600 MPa for 30 min at 30 degrees C) was investigated for different pressure release rates (20 to 600 MPa min-1) and casein contents (1 to 15 g/100 ml). Structures from liquid (sol) to solid (gel) were observed. The higher the protein content and the pressure release rate, the higher was the dynamic viscosity. A firm gel was built up at a casein content of 7 g/100 ml for a pressure release rate of 600 MPa min-1, while lower release rates resulted in less firm gels (200 MPa min-1) or liquid structures (20 MPa min-1). In a 5 g/100 ml casein solution and at a pressure release rate of 600 MPa min-1, casein aggregates were generated which were built from smaller casein particles with a larger hydrodynamic diameter and higher voluminosity than in the untreated solution. After a slow release rate casein micelles had a smaller hydrodynamic diameter and a lower voluminosity, but were similar in shape and diameter as compared with the micelles in solution before high pressure treatment.