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A number of components of essential oils, such as menthol, α and β pinene, guaiacol and eucalyptol and the oil of Pinus pumilio, were studied to establish if they affect the metabolism of other drugs in rats. Eucalyptol, administered subcutaneously or by aerosol was found to increase the in vitro (9000 g) liver metabolism of aminopyrine, p-nitro-anisol and aniline and in vivo the metabolism of pentobarbital.

A linear correlation has been obtained between sleeping time and brain pentobarbital concentration 90 min after treatment of rats with 30 mg/kg i.p. of pentobarbital. Pretreatment with various drugs abolished or modified this correlation. The effect of these drugs is analysed in an attempt to identify the mechanism responsible for the modification of the pharmacological response. SKF-525A, DDT and low doses of chlorpromazine seem to act on pentobarbital sleeping time only by modifying its metabolism. Amphetamine, on the contrary, reduces the pentobarbital sleeping time by a mechanism which is not of metabolic origin. With phenobarbital pretreatment there is a modification of pentobarbital sleeping time which is the resultant of two different mechanisms: increased metabolism and increased sensitivity. Chlorpromazine at high doses, as well as diazepam, affect pentobarbital sleeping time without affecting pentobarbital metabolism.