Introduction: Neuronal catecholaminergic activity modulates central nervous (CNS) function. - Specifically - Noradrenaline (NA) can exert a tuning or biassing function, whereby the signal-to-noise ratio is altered. Dopamine (DA) activity may promote switching between inputs and outputs of information to specific brain regions. Background: It has been ten years since evidence for a tuning function was advanced for NA (Segal & Bloom, 1976 a, b), and in the last five years the switching hypothesis for DA has been tentatively put forward (Cools, 1980). Review: Recent studies are reviewed to show that while catecholamine activity contributes to neural interactions in separate brain regions, that give rise to the organization of different functions, their working principles may be common between species and independent of the nucleus of origin. Behavioral examples are discussed and an attempt is made to integrate this with evidence from intracellular recording studies. It is suggested that the tuning principle in NA systems is particularly important for the formation of associations and neural plasticity (interference control), and that the switching principle of DA systems modulates the timing, time-sharing and initiation of responses (program-control).