Prostaglandin F2alpha was measured by means of radiommunoassay in the serum of women in the active phase of term labour before and after rectal indomethacin administration. Samples of blood were collected every 20 or 30 seconds, between and during contractions. There was a significant decrease of the level of PGF2alpha in plasma 30 to 30 minutes after indomethacin administration, but the pattern of rapid short-term fluctuations in prostaglandin levels was unaltered.