Background and purpose: While remifentanil can be used either during labour or fetal surgery, more should be known about the transplacental transfer of this opioid. The aim of this study was to investigate the placental transfer and haemodynamic effects of remifentanil after i.v. administration to pregnant ewes. ) for 1 h, and maternal and fetal arterial blood samples were drawn at regular intervals during and up to 1 h after the discontinuation of the infusion. Haemodynamic parameters were monitored continuously. Blood gas samples were drawn at baseline and once during the infusion. Key results: Peak maternal remifentanil plasma levels of 4.0 Ϯ 0.9 ng·mL -1 (mean Ϯ SD) and peak fetal plasma levels of 0.4 Ϯ 0.3 ng·mL -1 were obtained. Remifentanil plasma levels dropped rapidly after the discontinuation of the infusion. The continuous infusion of remifentanil did not result in significant maternal or fetal haemodynamic changes. Conclusions and implications:Remifentanil rapidly passes through the placenta of pregnant ewes and although remifentanil concentrations stay significantly lower in the fetus compared with those in the mother, remifentanil can be detected in significant amounts in the fetus.