The concentrations of plasma oxytocin and prostaglandin F2 alpha metabolite (PGFM) were measured in 10 parturients with and 10 without lumbar epidural analgesia. A blood sample was taken immediately before analgesia and another 60 min later. The control patients were matched for the stage of cervical dilatation at the time of the first blood sample; the second was drawn 60 min later. Plasma PGFM decreased significantly after lumbar epidural anesthesia and increased in controls resulting in a highly significant difference between the groups (P < 0.005). Plasma oxytocin concentrations levels also changed in opposite directions in the two groups but the difference did not reach statistical significance (P < 0.1). Uterine activity increased in the controls and decreased in the analgesia group resulting in a significant difference between the groups (P < 0.05). All subjects delivered vaginally. The total duration of labor was longer in the analgesia group (7.8 +/- 1.0 h vs. 4.7 +/- 0.6 h; P < 0.05) as was the duration after analgesia (5.1 +/- 0.9 h vs. 2.5 +/- 0.8 h; P < 0.05), whereas the duration of the second stage was not significantly different. We conclude that lumbar epidural anesthesia results in suppression of PGF2 alpha release which may be the cause of the diminished uterine activity and the prolonged duration of the first stage of labor.