The alpha(1) subunit provides both the voltage-sensing mechanism and the ion pore of voltage-dependent calcium channels. Of the six classes of alpha(1) subunit cloned to date, alpha)1A) is the subject of debate in terms of its functional correlate, although it is generally thought to encode voltage-dependent calcium channels of the omega-agatoxin IVA-sensitive, P/Q type. In the present study, an alpha(1A)-specific riboprobe and antibody were used with in situ hybridisation and immunohistochemical techniques to localise alpha(1A) messenger ribonucleic acid transcripts and subunit protein throughout the mature rat brain. Dual localisation of alpha(1A) protein and markers for acetylcholine, catecholamines, and 5-hydroxytryptamine have also been performed in a number of discrete areas. Abundant and widespread distribution of alpha(1A) protein was found, with immunoreactivity occurring both in cell bodies and as punctate staining in areas of neuronal processes. Several associations were noted across alpha(1A) localisation, defined neuroanatomical regions, and neurotransmitter systems. However, alpha(1A) expression was not confined to loci corresponding to any one neurotransmitter type, although a high level of expression was observed in cholinergic neurones. The distribution of the alpha(1A) subunit in the rat corresponded well with the limited human mapping data that are available.