The two New Zealand tea-tree oils, Manuka (Leptospermum scoparium J.R. et G. Forst) and Kanuka (Kunzea ericoides (A. Rich) J. Thompson), Myrtaceae have been used as folk medicines for treating diarrhoea, colds and inflammation but their pharmacological action has not been investigated. Their mode of action was therefore studied on the field-stimulated guinea-pig ileum. Both Manuka and Kanuka oils induced a spasmolytic effect but Kanuka produced an initial contraction. The spasmolytic action of both oils was the result of a post-synaptic mechanism. Action involving adrenoceptors or cGMP was not considered likely, neither did the oils behave like calcium- or potassium-channel openers. There is some evidence that Manuka acts through cAMP whereas the mode of action of Kanuka is as yet undetermined. The results indicate that the use of these oils as relaxants in aromatherapy might be valid, although their mode of action is not identical.