PIP: Unbound plasma cortisol was measured by 4 different methods in 31 pregnant women, in 18 women taking 50 mcg ethinyl estradiol and .5 mg norgestrel, and in 25 controls. The assay methods were total cortisol by fluorometry, transcortin by gel filtration, unbound cortisol by dialysis with carrier tritiated cortisol and serum albumin by colorimetry with methyl orange. Unbound cortisol was calculated theoret ically based on total cortisol, transcortin, albumin, and their dissociation constants for cortisol; derived experimentally by dialysis; and also calculated from all of the above data. All results expressed in mcg/100 ml were significantly higher in second and third trimester pregnant women and in those using oral contraceptives. For example, calculated unbound cortisol was 1.40, 2.20, and 1.33 in these groups compared with 1.09 mcg/100 ml in controls. Unbound cortisol expressed as a percentage of total cortisol was lower in these 3 groups: 4.05, 4.63 and 3.43 percent compared with 6.36 percent. The sum of unbound cortisol and cortisol bound to albumin was significantly lower in these 3 groups, 10.4, 11.8, and 10.2 percent compared with 19.2 percent in controls, not different in midtrimester pregnancy, but higher, 5.64 mg/100 ml in third trimester and 3.96 mcg/100 ml in pill users, than in controls, 3.31 mcg/100 ml.