For a group of normal medical students, we examined the effect of posture on the concentration of a number of constituents in plasma. On standing, there is a significant increase in plasma total protein, albumin, calcium, alkaline phosphatase, bilirubin, and cholesterol--all proteins or substances bound to protein. Although it is possible to make an allowance for postural variations in plasma calcium, no correction is possible for changes in protein concentrations and care is needed whenever the precise values are important, as in the follow-up of patients with the nephrotic syndrome.