In an earlier study, we examined the pressure-response characteristics of rat aortic baroreceptors with C-fibre (non-medullated) afferents. Compared with aortic baroreceptor fibres with A-fibre (medullated) afferents, the C-fibres were activated at higher pressures and discharged more irregularly when stimulated with a steady level of pressure. Here we examine the relationship between discharge and the aortic diameter in these two types of afferents in rats and rabbits. An in vitro aortic arch/aortic nerve preparation was used to record single-fibre activity simultaneously with aortic arch pressure and diameter. Diameter was measured using a highly sensitive non-contact photoelectric device. Baroreceptor discharge was characterized by stimulating the nerve endings with either slow pressure ramps from subthreshold to 200-250 mmHg, at a rate of rise of 2 mmHg s-1, or pressure steps from subthreshold to suprathreshold levels, at amplitudes of 110-180 mmHg. In response to these inputs, C-fibres in rabbits (conduction velocities= 0.8-2.2 m s-1) behaved much like those in rats. The C-fibres had significantly higher pressure thresholds (95 +/- 3 mmHg vs. 53 +/- 2 mmHg; mean +/- SEM), lower threshold frequencies (2.4 +/- 0.5 vs. 27.7 +/- 1.8 spikes s-1), lower maximum discharge frequencies (22.7 +/- 2.3 vs. 65 +/- 5.8 spikes s-1) and more irregular discharge in response to a pressure step when compared with A-fibres (conduction velocities of 8-16 m s-1). When plotted against diameter, C-fibre ramp-evoked discharge increased gradually at first, and then rose steeply at increasingly higher ramp pressures where aortic diameter became relatively constant. In contrast, A-fibre discharge was linearly related to diameter over a wide range of pressure. These results suggest two interpretations: (1) The relation between stretch and C-fibre discharge is highly non-linear, with a marked increase in sensitivity at large diameters. (2) C-fibres are stimulated by changes in intramural stress rather than stretch.