The measurement of osmotic pressure in blood and urine has been used extensively in clinical investigation and is beginning to be available as a diagnostic test in the hospital laboratory [1–4]. It is dependent on the number of solute particles in solution, rather than their shape, weight, or charge. The determination of the freezing point depression of a solution gives a good estimate of the number of particles and is the basis for the calculation of the osmolality. Serum osmolality has a normal range of 275 to 295 mOsm/kg, with electrolytes, particularly sodium, contributing over 90 percent of this value. Glucose, nonprotein nitrogen, and protein substances make up a large portion of the remainder. The average osmolality of urine ranges from 300 to 1090 mOsm/kg, varies with dietary intake, and is generally higher in males [3]. The mean value for cerebrospinal fluid osmolality in fifty living patients was found to be 281 mOsm/kg, and the range of values extended from 269 to 304 mOsm/kg [5]. Postmortem spinal fluid studies to date have not included osmolality determinations.