BACKGROUND The aim of this study was to determine the lowest effective bolus dose of oxytocin to produce adequate uterine tone (UT) during elective Caesarean delivery (CD). METHODS Seventy-five pregnant patients undergoing elective CD under spinal anaesthesia were randomized to receive oxytocin (0.5, 1, 3, 5 units) or placebo. UT was assessed by a blinded obstetrician as either adequate or inadequate, and using a verbal numerical scale score (0-10; 0, no UT; 10, optimal UT) at 2, 3, 6, and 9 min after oxytocin administration. Minimum effective doses of oxytocin were analysed (ED(50) and ED(95)) using logistic regression. Oxytocin-related side-effects (including hypotension) were recorded. RESULTS There were no significant differences in the prevalence of adequate UT among the study groups at 2 min (73%, 100%, 93%, 100%, and 93% for 0, 0.5, 1, 3, and 5 units oxytocin, respectively). The high prevalence of adequate UT after placebo and low-dose oxytocin precluded determination of the ED(50) and ED(95). UT scores were significantly lower in patients receiving 0 unit oxytocin at 2 and 3 min compared with 3 and 5 units oxytocin (P<0.05, respectively). The prevalence of hypotension was significantly higher after 5 units oxytocin vs 0 unit at 1 min (47% vs 7%; P=0.04). CONCLUSIONS The routine use of 5 units oxytocin during elective CD can no longer be recommended, as adequate UT can occur with lower doses of oxytocin (0.5-3 units).