Objective: verification of a hypothesis assuming that 5-HT3 receptor blockade by intravenous administration of ondansetron reduces the incidence of hypotension and bradycardia in patients undergoing spinal anaesthesia for Caesarean section. Material and methods: The study design was approved by the Bioethics Committee and included 72 patients undergoing elective Caesarean section, randomly assigned to ondansetron group (group O) or placebo group (group P). Finally, group O encompassed 35 patients administered ondansetron 8 mg i.v. dissolved in 10ml 0.9% NaCl whereas group P consisted of 34 patients receiving 0.9% NaCl 10 mg. Systolic and diastolic pressures were measured every 2 minutes since the onset of anaesthesia. Heart rate (HR) was monitored continuously. The criterion of hypotension requiring ephedrine was a decrease in systolic pressure by 20% compared to its baseline value or a decrease in systolic pressure below 90 mm Hg. The criterion of bradycardia was a decrease in HR below 60/min. Results: Hypotension was observed in 14 group O patients (39%) and in 15 group P patients (44%); the difference was not statistically significant. Bradycardia was noted in 1 group O patient (3%) and in 2 group P patients (6%); the difference was not statistically significant. Conclusion: A hypothesis assuming a reduction in pressure following subarachnoid anaesthesia for Caesarean section after the administration of 8 mg of ondansetron was not confirmed.