Since our student days we have been accustomed to enumerating the individual elements of the heart's rhythmic centre and conduction system in accordance with the order of their functions, from cranial to caudal. We begin, therefore, with the sino-atrial node in the right atrium, after which comes the atrioventricular node, the bundle of His and, finally, Purkinje's fibres. The respective discoveries were made in precisely the opposite order, however. Purkinje's fibres were discovered in 1839, the bundle of His in 1893, the atrioventricular node was discovered by Aschoff and Tawara in 1905-1906 and the sinus node by Keith and Flack in 1906. The biographies of the respective discoverers and the circumstances surrounding their discoveries are dealt with briefly.