Muscarinic receptors were localised on cells cultured from the atria and interatrial septum of newborn guinea pig heart by autoradiography using [3H]propylbenzilylcholine mustard. All of the intracardiac neurones observed in these cultures were specifically labelled: both the neuronal cell body and processes were evenly labelled over their entire surface. Autoradiographic grains were also uniformly distributed over atrial myocytes in culture. This even pattern of labelling of both neurones and atrial myocytes was not changed by the substitution of serum-supplemented growth medium with serum-free, hormone-supplemented, defined medium. The demonstration of muscarinic receptors on intracardiac neurones has important implications for studies on the roles of these neurones in the mammalian heart.