1. The binding of cortisol to plasma proteins has been studied by multiple equilibrium dialysis of plasma samples taken from 14 patients before treatment after moderate or severe accidental injury and 2–5, 6–12 and 14–25 days later.
2. The total plasma cortisol concentration was initially high, but fell by 6–12 days to stable values probably near the upper limit of normal.
3. The plasma albumin and total protein concentrations fell during the first few days after the injury, but the fall in albumin was proportionally the greater. Both were rising again at the end of the study period. The albumin concentration was positively correlated with an index of its binding to cortisol calculable from the dialysis data.
4. The concentration of corticosteroid-binding globulin (CBG) rose between weeks 1 and 2 after trauma, with no statistically significant change before or afterwards.
5. The changes in binding proteins after injury were relatively small and did not greatly alter the relationship between free and total cortisol concentrations in plasma. It is concluded that in most trauma patients changes in free cortisol, generally considered to be the active fraction in plasma, can be followed adequately by measuring the total cortisol concentration.