A simple method for the determination of free cortisol in plasma, with normal levels in men and women, including women taking low oestrogen contraceptives.

A small scale equilibrium dialysis method for the measurement of the non-protein bound cortisol in plasma is described. Dialysis is carried out in PTFE cells against an isosmolar Hepes buffer, equilibration occurring within 18 hours. Inter-assay variation was found to be approximately 6%. Samples taken from normal males and females and from oestrogen-treated females were analysed and the results compared with published levels. A significant difference in both the total and free cortisol levels was found between the normals and oestrogen-treated females.