Recently, ketanserin (serotonin receptor antagonist) has been reported to be beneficial in the treatment of preeclampsia. In this study, we investigated the effects of ketanserin on the placental blood flow, fetal weight and placental weight in spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto rats (WKY). We measured the rats’ placental blood flow by monitoring the clearance of hydrogen gas generated by electrolysis. In ketanserin (12, 24, 48 mg/kg)-treated SHR, systolic blood pressure was significantly decreased. The placental blood flow was significantly (p < 0.05) reduced compared to that before treatment in WKY and SHR. The fetal weight decreased dose dependently in WKY, while in SHR it did not decrease significantly. The placental weight of ketanserin-treated rats decreased compared to that before treatment. These data suggest that ketanserin might have some reducing effects on placental blood flow, fetal weight and placental weight proportional to the dose.