The spectrum of effects that constitutes the state of general anaesthesia can be induced by the combined use of drugs. Each drug has a predominant action regarding one of the anaesthesia components, hypnosis, analgesia and amnesia. However, each agent, when used in combination, not only produces its own expected effect, but it can also modify the effect of another agent acting on a different component. For example, an opioid, in addition to its anti-nociceptive effect, can also potentiate the hypnotic effect of a benzodiazepine. Anaesthetists have long recognized these effects but did not quantify them until recently. Pharmacologists have provided us with techniques to measure drug interactions. We have utilized these techniques to demonstrate and quantify significant pharmacological interactions for hypnotic effect with commonly used intravenous agents. The clinical utility of these combinations can now be exploited precisely for the benefit of all our patients.