The phenomenon of resetting and the possible mechanisms responsible for it were investigated in aortic baroreceptors having unmyelinated axons. Recordings were obtained from 31 aortic baroreceptor C-fibers in 20 normotensive Wistar Kyoto rats (WKY) and 37 baroreceptor C-fibers in 11 spontaneously hypertensive rats (SHR). The recordings were obtained from the left aortic nerve of an in vitro aortic nerve-aortic arch preparation. The thresholds for activation of baroreceptor C-fibers were not different between SHR's and WKY's at 16 weeks of age (139 +/- 7 and 133 +/- 5 mmHg, respectively). However, the thresholds for activation in SHR at 36 weeks of age were considerably higher (163 +/- 4 mmHg) indicating resetting of the receptors in the older rats. The pressure-discharge curves were not different between SHR's and WKY's at 16 weeks of age and WKY's at 35 weeks of age, but were shifted to the right for SHR's at 36 weeks of age. The receptors showed greater responses to pulsatile pressures than to steady state pressures at the same mean level of pressure. We found that aortic C-fibers were not reset during the early established phase of hypertension in SHR's because of enhanced receptor strain sensitivity. This is likely to be of importance for tonic vasomotor control at this time. In later stages of hypertension and despite enhanced strain sensitivity the endings were reset to higher pressures probably due to an increased collagen content in the aortic wall.