BACKGROUND This study examined the effect of different levels of spinal anaesthesia, induced by solutions of different baricity but containing the same amount of local anaesthetic agent, on the requirement for sedation with propofol. METHODS Thirty-six patients undergoing varicose vein surgery under spinal anaesthesia were randomly allocated to receive tetracaine 15 mg in 3 ml of either glucose 5% (hyperbaric) or CSF (isobaric). I.V. propofol was started 5 min after the intrathecal injection and was titrated to maintain a bispectral index (BIS) score of 65-75. The propofol requirements to maintain this range in the two groups were compared every 5 min. RESULTS The propofol requirement was always lower in the hyperbaric group, with the differences becoming statistically significant 20 min after the intrathecal injection. Total consumption of propofol over the 55 min of the study was also less in the hyperbaric group. CONCLUSION The known difference in level of spinal anaesthetic block induced by solutions of different baricity, but the same dose of local anaesthetic, was associated with different requirements for propofol sedation as determined by BIS assessment.