Oestriol and progesterone concentrations were measured by specific radioimmunoassays in the saliva of six women on a daily basis during the last month of pregnancy, at frequent intervals during labour and daily during the puerperium. Salivary steroid concentrations are thought to reflect the circulating concentrations of the free hormone, and hence may be more biologically relevant than the total plasma concentration, or the urinary excretion of a metabolite. There was no sign of a fall in salivary oestriol or progesterone concentrations before the onset of labour. During the first and second stages, the concentrations fluctuated widely, perhaps due to alterations in uterine blood flow. After delivery of the placenta, hormone concentrations declined abruptly. Oestriol reached undetectable levels of less than 0.1 nmol/l within a day, whereas progesterone concentrations declined somewhat more slowly, approaching follicular-phase values after 2-3 days. This slower decline in the concentration of progesterone may be due to a leaching-out of the hormone from body fat stores. The absence of any prepartum progesterone withdrawal presumably explains the characteristic postpartum delay in the onset of lactogenesis.