Plasma samples were obtained at weekly intervals from the peripheral circulation of 12 women in the last 2-7 weeks of pregnancy. The concentrations of oestradiol and progesterone (isolated by chromatography) were measured by radioimmunoassay; the proportion of each hormone which was not bound to protein was measured by steady-state gel filtration. From these, the apparent concentration of the non-protein-bound form of each hormone was calculated. The mean proportion of oestradiol not bound to protein varied from 0.84 to 2.71% in the different subjects, but within each subject variation was within experimental error. For progesterone, the mean proportion not bound to protein in the different subjects varied from 1.76 to 2.77%; within individuals the proportion remained essentially constant. There was no consistent, recognizable trend as labour approached in the concentration of oestradiol; the concentration of progesterone; the concentrations of non-protein-bound oestradiol or non-protein-bound progesterone; the ratio of the concentrations of progesterone and oestradiol; the ratio of the concentrations of non-protein-bound progesterone and oestradiol. In nine out of 12 subjects, the ratio of the concentration of non-protein-bound progesterone to that of non-protein-bound oestradiol was greater than the corresponding ratio based on total hormone concentrations. These results therefore provide no support for the hypothesis that human labour is preceded by alteration in the progesterone to oestradiol ratio which can be detected by measurement of these hormones in peripheral blood.