Synaptophysin is a presynaptic vesicle protein and a marker of synaptic density. We have studied the expression of its encoding mRNA in the brains of rats treated with haloperidol (2 mg kg-1 d-1) for two weeks. A significant increase in synaptophysin mRNA content was observed in the dorsolateral striatum but not in other brain areas compared with control animals. A similar trend was observed for synaptophysin immunoreactivity. Quantification of synaptophysin mRNA per cell showed that the increase was pronounced in large putatively cholinergic, striatal neurones. These data provide further evidence that localized synaptic plasticity occurs after neuroleptic treatment and indicate that such alterations are manifested in terms of expression of a synaptic protein gene by striatal neurones.