Mammalian intrinsic cardiac neurons subserve different functions in different cardiac regions, but the regional anatomical organisation of the intracardiac nervous system is not well understood. We investigated the quantitative and qualitative distribution of cholinergic and adrenergic elements, and the intracardiac pathways of extrinsic cardiac nerves, in whole-mount preparations of guinea pig atria. Protein gene product 9.5 immunoreactivity (PGP 9.5-IR) marked intracardiac neuronal elements; immunoreactions for choline acetyltransferase (ChAT-IR) and tyrosine hydroxylase (TH-IR) distinguished cholinergic and adrenergic components, respectively. Catecholamine-containing components were identified by aldehydeinduced fluorescence histochemistry. Mean total number of atrial neurons was 1510 ฯฎ 251 (SE); 85% of these occurred in ganglia of ี
20 neurons. All neuronal somata expressing PGP 9.5-IR also expressed ChAT-IR, suggesting that these neurons were cholinergic. Right (RA) and left (LA) atria had statistically similar neuronal densities (6.4 ฯฎ 1.2 and 2.4 ฯฎ 0.7 neurons/mm 2 , respectively; analysis of variance, Pี
0.05). Neurons in RA were concentrated intercavally; LA neurons were concentrated near pulmonary vein ostia. Greatest density occurred in the interatrial septum (16.3 ฯฎ 4.0 neurons/mm 2 ). No neuronal somata expressed TH-IR or contained detectable amines but these elements were expressed by somata of small cells (mean total 124 ฯฎ 33) throughout the atria, primarily associated with ganglia. Amineand TH-containing varicosities were also present in ganglia, representing potential sites for adrenergic modulation of ganglionic neurotransmission. Branches of extrinsic cardiopulmonary and vagus nerves were distributed to all parts of both atria. The organisation of the intracardiac nervous system revealed in this study will facilitate further investigations of regional autonomic control of the heart.