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SummaryLocal anaesthetic solution is sometimes used to enlarge the epidural space in preparation for the insertion of an indwelling epidural catheter and during subsequent checking a small volume of fluid may be aspirated. Twelve anaesthetists were asked to distinguish 0.1 ml plain bupivacaine 0.25% from 0.1 ml simulated cerebrospinalfluid (CSF) solution. The distinction was based on differences in the physicochemical properties of the two test solutions, namely, temperature, presence of glucose, pH, and turbidity when mixed with thiopentone. Temperature difference resulted in correct identification of the two solutions in 75% of cases, the presence of glucose in 92%, pH in 83%, and turbidity in 25% of cases. No single test resulted in correct identification by all the anaesthetists. We suggest that more than one physicochemical test is required to identifv correctly the nature of the small volume of liquid which may be aspirated from an epidural catheter.