Serum 5 alpha-pregnane-3,20-dione (5 alpha DHP) and progesterone (P) concentrations before spontaneous labor were studied in 13 women whose pregnancy and delivery were uncomplicated. Blood samples were drawn twice weekly from the 36th week of gestation up to the onset of spontaneous labor. P and 5 alpha DHP values were analysed by applying RIA methods. Mean serum 5 alpha DHP concentrations declined significantly during the last week of pregnancy, from 135 +/- 12 nmol/l 5 days prior to spontaneous labor, to 111 +/- 10 nmol/l on the day spontaneous labor started (p less than 0.005). No significant changes in progesterone concentration occurred during the observation period. These results suggest a change in 5 alpha DHP production preceding the onset of spontaneous labor.