This study examined firing patterns of single-fiber carotid baroreceptors in response to slow ramp increases in carotid sinus pressure (1-2 mm Hg/sec) in vascularly isolated carotid sinus preparations in thiopental-anesthetized dogs (25 mg/kg, plus 10 mg/kg/hr). Two general types of baroreceptor discharge patterns were obtained: 1) type I, a discontinuous, hyperbolic pattern characterized by a sudden onset of discharge at threshold pressure with a relatively high threshold frequency, which gradually increased to a higher saturation firing rate and 2) type II, a continuous, sigmoidal pattern characterized by a gradual increase in discharge above threshold pressure and a relatively low threshold frequency and saturation firing rate. Type II baroreceptor curves typically showed spontaneous discharge below threshold pressure and significantly lower sensitivities, threshold frequency, saturation firing rate, and threshold pressure than those of type I receptors. However, the saturation pressures and operating ranges of the type II receptors were greater than those of type I receptors, and the pressure at which type II receptors had their greatest sensitivity was greater than that for type I receptors. Type I baroreceptors generally had large myelinated afferent A fibers; type II baroreceptors generally had smaller A and unmyelinated C fibers, based on conduction velocities. The presence of spontaneous activity with type II baroreceptors, combined with significantly lower sensitivities and wider pressure operating ranges seen relative to type I baroreceptors, suggests that these receptors may primarily serve to provide information on tonic, or baseline, levels of arterial blood pressure to the central cardiovascular centers. The sudden onset of discharge, higher sensitivities, and narrower operating ranges of type I baroreceptors suggest that their primary role may be to provide information on dynamic, sudden changes in arterial pressure.