In a double-blind study 28 patients with acute, localised muscle pain received four local injections of mepivacaine 0.5%, and 25 patients with the same type of pain received local injections of an equivalent volume of physiological saline. The group receiving saline tended to have more relief of pain, especially after the first injection. The results thus show that pain relief is not due merely to the local anaesthetic. The study therefore raises questions about the mechanism by which local injections into muscle relieves pain, since there is the possibility that a similar effect might also be achieved by merely inserting a needle into the trigger point. Physiological saline is considered to be a more appropriate fluid for injection therapy than local anaesthetics since it is less likely to produce side-effects.