The characteristics of 47 non-medullated and 54 medullated fibres arising from aortic arches of 6 hypertensive rabbits have been investigated. The threshold for activation of the aortic C-fibres lay between 78 and 190 (mean 122) mmHg. The threshold for activation of 54 medullated aortic baroreceptors in the same animals was from 70 to 140 (mean 109) mmHg. At the awake mean arterial blood pressure (MABP) of these hypertensive rabbits (143 mmHg) all the medullated fibres were active as were 78% of the C-fibres. In contrast in a group of normotensive animals 91% of the medullated and only 28% of the non-medullated fibres were active. Pressure response curves were constructed for 19 C-fibres and 12 medullated fibres from the hypertensive animals. At a MABP of 110 mmHg the firing in the non-medullated fibres was 2.0 Hz as compared with 19.0 Hz in the medullated fibres and at the awake MABP mean firing was 9.0 Hz in the non-medullated fibres and 48.0 Hz in the medullated fibres. The firing at awake MABP was considerably higher in the hypertensives (9.0 Hz) as compared with the normotensives (1.1 Hz). Thus these data indicate that although the aortic C-fibres are reset in chronic hypertension, they are reset less than the medullated aortic baroreceptors from the same animal. It is suggested that arterial baroreceptor C-fibres may have an important role in the tonic control of the circulation in hypertension.