The aim of this study was to see whether leukotrienes are present in human umbilical plasma at birth either before or after labour. Plasma concentrations of LTB4 (6.02 ng/ml) and LTD4 (6.11 ng/ml) were significantly higher (P less than 0.01) after spontaneous vaginal delivery than during elective caesarean section before labour (LTB4 = 1.96 ng/ml, LTD4 = 0.18 ng/ml). A possible involvement of leukotrienes in the biochemical and metabolic events of human labour is suggested.