In order to ‘dissect out’ the function of a given region of the brain, the local application of a chemical transmitter probably parallels closely what is actually occurring biologically at the local site. This paper addresses itself to the problem of diffusion of drugs and describes the application of a modern device (the osmotic minipump) which produces a constant flow of minute quantities of the agent into the brain structure. Neurophysiological, behavioural, and histochemical investigations have been carried out in different species. Although the histochemical results of the minipump experiments indicate some spread, yet this seems to be at a concentration which would have no apparent pharmacological action. This was confirmed by our neurophysiological and behavioural studies and also by some recent reports in the literat