Abstract: Rapid colloidal osmometer 10 for readout of osmotic equilibrium pressure comprising a cap of rigid semipermeable ceramic 12, a solvent chamber 14 and a pressure transducer 18. The osmometer is for insertion into a living subject. Osmotic equilibrium pressure is calculated by a data reduction computer 40 and displayed on a chart recorder 42. The computer can also be used to control fluid infusion through a valve 44 into a patient 29. An alternate embodiment comprises an osmometer for high pressure use in a pressure chamber 54 in which the osmometer 56 is braced 58. A solution chamber 57, semipermeable membrane 32, and solvent 55 are held at a high pressure by compressor 78. Osmotic equilibrium is predicted by a computer 40 and displayed on a chart recorder 42.