To find out whether a serum factor in pre-eclamptic women is the cause of the raised serum endothelin concentrations associated with the disorder, the effect of serum from pre-eclamptic women on endothelin production by endothelial cells was compared with that of serum from pregnant and non-pregnant controls. The finding that pre-eclamptic serum suppresses endothelin production suggests that it contains a factor that might be part of a homoeostatic response to raised serum endothelin concentrations in pre-eclampsia.