BACKGROUND: We investigated a possible effect of ondansetron on the duration of sensory and motor block produced by ropivacaine. METHODS: Fifty male patients undergoing transurethral surgery received either 8 mg oral ondansetron the evening before surgery plus IV 8 mg ondansetron 15 min before subarachnoid anesthesia or placebo. All patients received 2.2 mL of 0.75% plain ropivacaine intrathecally. Sensory and motor block were assessed 30 min after the intrathecal injection and every 30 min thereafter until recovery from the motor block. RESULTS: Thirty minutes after spinal injection of ropivacaine, we first measured, in both groups, the time to maximum block for both sensory and motor modalities. The maximum level of the sensory block, defined as decreased sensation, was T8 in the control and T6 in the ondansetron group, and absence of sensation was defined as T11 and T9 for the control and the ondansetron groups, respectively. Regarding block duration, 180 min after spinal injection, sensory block was detected in 11 of 22 and 16 of 24 patients and motor block in 1 of 22 and 0 of 24 in the control and ondansetron groups, respectively. Sensory and motor block did not differ between groups at any measured time point. CONCLUSIONS: Ondansetron had no effect on the subarachnoid sensory or motor block produced by ropivacaine.