Plasma concentrations of oxytocin in nine spontaneous labors (group 1), nine amniotomy-induced labors (group 2), six prostaglandin F2 alpha-induced labors (group 3), and five prostaglandin E2-induced labors (group 4) were determined at the stages of 1 to 7 days preceding the onset of labor, onset of labor, and established labor, by means of unextracted radioimmunoassay. The levels of oxytocin in the stage preceding the onset of labor in group 1 were not significantly different from the levels of oxytocin in the corresponding stages in groups 2, 3, and 4. However, the levels of oxytocin in the stages of onset of labor and established labor in group 1 were significantly higher than those in the corresponding stages in groups 2, 3, and 4. These results suggest that oxytocin plays a leading role in the onset and progress of spontaneous labor, whereas something other than oxytocin might be involved in the labor induced by amniotomy alone or by prostaglandins.