The daily amounts of salivary progesterone have been determined over the complete menstrual cycle for 9 normal women. The level of progesterone during the follicular phase was about 150 pmol/l and increased significantly to about 350 pmol/l during the luteal phase of the menstrual cycle. The amounts of salivary progesterone were significantly correlated with those in blood (P less than 0.001) in paired saliva-blood specimens taken from 96 women. Although these volunteers comprised patients with benign and malignant breast disease and normal unaffected women, the relationship between salivary and blood progesterone was similar for all groups. The concentration of non-protein-bound progesterone was determined using equilibrium dialysis. To correct for serum dilution the linear relationship between the percentage of progesterone bound and the reciprocal of serum dilution has been exploited. The values of non-protein-bound progesterone obtained were significantly and linearly correlated with levels in saliva (r = 0.75, P less than 0.001, d.f. = 34) although the amount of free progesterone in blood was about five times that found in saliva.