The cervical vagus consists of about 30 thousand fibres of which about 24 000 are sensory in function (Agostoni, Chinnock, Daly and Murray 1957). Of the latter about 3 thousand are myelinated and have been the centre of attraction in electrophysiological studies chiefly owing to the relative ease with which impulses can be recorded in them. In the 19th International Physiological Congress, Whitteridge reviewed briefly all that was known about these myelinated afferent fibres which arose from endings of various kinds in the heart and lungs (Whitteridge 1953) and although more detailed information about certain myelinated sensory endings has been gained since that time, the main advance has been in the direction of recording impulse in non-medullated fibres from endings in the gastro-intestinal tract [Paintal 1953 (d), 4954 (a), (b); Iggo 1955, 1957 (b)]. Till Iggo’s clear demonstration (Iggo 1958) that the fibres concerned are mostly non-medullated, it was generally imagined that unitary discharges in non-medullated fibres could be recorded only with great difficulty owing to their small size. It was felt that if recording impulses in single fibres of small myelinated fibres presented certain difficulties, it would be still more difficult to record impulses in nonmedullated fibres. However, as pointed out by Iggo (1958), the situation is probably not comparable because non-medullated fibres occur in small bundles which can probably be dissected as such. Since there are many more non-medullated afferent fibres (21 000) than medullated ones (3 000) (Fig. 1), this therefore opens up a practically unexplored field for future work.