Myelinated pulmonary afferents are classified as rapidly adapting receptors (RARs) or slowly adapting receptors (SARs) by their adaptation rate. Behavior of SARs varies greatly, and therefore the present study tries to further categorize SARs according to their mechanical properties. Single-fiber activity of 104 SARs was examined in anesthetized, open-chest, artificially ventilated rabbits. According to the increase or decrease in activity during removal of positive-end-expiratory pressure (PEEP), SARs were divided into two groups. In one group mean activity increased from 31 ± 6 to 46 ± 7 impulses per second (imp/s; n = 11); in another group mean activity decreased from 44 ± 2 to 25 ± 1 imp/s (n = 93). The first group of SARs has high adaptation indexes (RAR-like), which increased with inflation pressure (36 ± 3, 44 ± 3, and 47 ± 3% for 10, 15, and 20 cmH₂O, respectively; P < 0.005). Their peak activity shifted from inflation phase to deflation phase during PEEP removal. The second group of SARs has low-adaptation indexes (typical SARs), which were not affected by inflation pressure (19 ± 1, 18 ± 1, and 17 ± 1% for 10, 15, and 20 cmH₂O; P = 0.516). Their peak activity did not shift during PEEP removal. Because there are overlaps in other characteristics, it is proposed that myelinated vagal afferents are viewed as a heterogeneous group; their behaviors are like a spectrum, where typical RARs and SARs represent two extremes of the spectrum. The receptor behavior might be determined by anatomic location and its environment.