Concentrations of thromboxane B2 (TXB2) have been measured in amniotic fluid obtained during late pregnancy and labour. Samples taken at amniocentesis contained significantly lower levels of TXB2 than samples taken by amniotomy for the induction of labour which in turn contained significantly less than when the same procedure was performed during labour. In the latter group there was no correlation between the degree of cervical dilatation and the concentration of TXB2. There was no correlation between gestational age and the concentration of TXB2 in amniotic fluid obtained at amniocentesis whereas there was a significant correlation (P<0·025) between these parameters when fluid was taken by amniotomy before labour. It is suggested that the relative activities of the enzymes responsible for the conversion of prostaglandin endoperoxides to prostaglandins, thromboxane A, and prostacyclin may be of great importance during parturition.