An experiment was conducted on 8 cannulated dairy goats which received four different mixed diets of lucerne hay and concentrate. On a dry matter (DM) basis, the two high forage diets contained 55 per cent lucerne hay, 10 per cent sugar beet pulp and 35 per cent concentrate, while the two low forage diets consisted of 30 per cent lucerne hay, 35 per cent sugar beet pulp and 35 per cent concentrate. The compound feed consisted of 74 per cent either barley (a rapidly degradable source of starch) or maize (a slowly degradable one). Each goat received each diet according to a latin square design. During each period, samples of rumen juice were taken before and 2, 4, 6 and 8 hours after the morning feed. Osmotic pressures were similar between diets (about 300 mOsm per kg H2O) before the morning feed. They were significantly increased by the barley diets during the 8 hours following the meal. A peak of 350 mOsm per kg H2O was observed for barley diets 2 hours after feeding, thereafter the osmotic pressures decreased to a plateau of about 320 mOsm per kg H2O for the rest of the sampling period. With the maize concentrate, osmotic pressure did not vary significantly for the high forage diet, and slightly increased (10 per cent) for the low forage diet. Thus, the kinetics of osmotic pressure can be significantly modified by diet composition.