The nucleus accumbens (N.Acc.) has been ascribed an important role in mediating locomotor activity and exploration, as well as more complex processes such as reinforcement, reward and the acquisition of displacement activities. Previous investigations of N.Acc. functions have primarily been based on pharmacological manipulations of activity of one of the main neurotransmitters in the N.Acc., dopamine (DA), either through administration of dopaminergic agonists or antagonists or through depletion of DA terminal fields in the N.Acc. In the present thesis, the functional role of the N.Acc. in a number of different forms of behaviour has been investigated further using specific, fibre-sparing excitotoxic lesions of intrinsic neurones, intra-accumbens injections of DA and in vivo electrochemical measurements of extracellular levels of DA in the N.Acc. Excitotoxic lesions in the N.Acc. were found to enhance spontaneous locomotion and exploratory behaviours while leaving intact the locomotor- stimulating effects of an indirect dopaminergic agonist, displacement drinking in response to intermittent food-reinforcement (SIP) and amphetamine-induced conditioned place preference (CPP). Thus, fibre-sparing excitotoxic lesions induced a pattern of behaviour distinct from that observed following terminal depletion in the N.Acc. Further, microinjection and in vivo electrochemical experiments showed no direct relationship between DA activity in the N.Acc. and SIP. Overall, these results are discussed in terms of a theoretical model proposing that the N.Acc. may function as an interface between sensory input and locomotor output and that inhibitory activity in the N.Acc. is needed to channel activity levels appropriately in response to cortical input about the direction of change. It is suggested that rather than viewing it as a unitary structure with specific functions, the N.Acc. should be considered as a heterogeneous part of the striatal complex with a number of distinct subsystems that exist within a complex framework of interactive processes, where changes in one structure can only be understood by taking into account other, related structures.