In the past, studies relating neurotransmitter function to behaviour have involved looking at the behavioural effects of specific drugs or lesions, or using post-mortem biochemistry in animals killed after undertaking the behaviour. Whilst these methods still play an important part in behavioural pharmacology research, recent advances in the technology of in vivo methods have enabled direct measurements of neurotransmitter function to be made in conscious and unrestrained animals. Among the more important of these methods are voltammetry and brain wash methods (e.g. push-pull perfusion, cortical cup) /116/. However, perhaps the greatest impact on this field of research has been made by microdialysis. This article reviews the contribution made by microdialysis to the understanding of neurotransmitter systems underlying normal physiological function and behaviour, and points to possible future directions for this work.