Ouabain was recently isolated from human plasma, bovine hypothalamus and bovine adrenal in attempts to identify endogenous substances inhibiting the cell membrane sodium pump. A number of radioimmunoassays have been developed in order to study the clinical significance of ouabain. The results have been controversial with regard to the presence and chemical nature of plasma ouabain-like immunoreactivity. We have now measured ouabain in healthy and pregnant individuals using solid-phase extraction of plasma samples followed by a new radioimmunoassay with the extraordinary sensitivity of at least 2 fmol/tube (5 pmol/l). Plasma extracts, a previously isolated human plasma ouabain-like compound and bovine hypothalamic inhibitory factor displaced the tracer in parallel and eluted identically with ouabain in highperformance liquid chromatography. Plasma ouabain immunoreactivity was found to be much lower than reported previously: 12·6 ± 1·3 pmol/l in healthy men (mean ± S.E., n=20) and 9·4 ± 0·7 pmol/l in women (n=14). In pregnant women (n=28) plasma ouabain concentration was 16·3 ± 4·0 pmol/l during the first trimester, 18·8 ± 4·3 pmol/l during the second trimester and 24·3 ± 4·0 pmol/l during the third trimester (all P<0·01 compared with non-pregnant women). Plasma ouabain 3-5 days after the delivery was 13·6 ± 1·1 pmol/l (n= 10, P<0·05-0·01 compared with second and third trimesters). The pregnancy-related changes in the plasma concentrations of ouabain resembled those of cortisolo. Therefore cortisol was measured from the same plasma samples and a significant positive correlation was found (r=0·512, P=0·006). The similar profiles of plasma ouabain and cortisol during pregnancy and their rapid decreases postpartum are consistent with the adrenal cortical origin of ouabain and also show that the secretions of these hormones are possibly under the control of same factors.