Human heart-lung transplantation recipients (HL) are models for the effects of selective lower respiratory tract denervation. We used a series resistor to examine thresholds of inspiratory flow resistance perception among 16 HL in comparison with 15 normal control subjects (C) and 12 heart transplant recipients (H). HL and C were closely age matched. H subjects were older and had mildly reduced lung volumes compared with the other groups. The perception threshold was defined as the added resistance (delta R) detected by subjects at 50% of random, blinded presentations. These delta R were subsequently expressed as a fraction of the airway resistance of the subject and baseline resistance of the apparatus (delta R/Raw + Rapparatus), otherwise known as the Weber fraction. The mean +/- SEM Weber fractions were nearly identical for HL and C (0.32 +/- 0.05 versus 0.34 +/- 0.05, respectively). The Weber fraction of H (0.74 +/- 0.17) was significantly greater than those of both other groups (p less than 0.05 by Kruskal-Wallis test). We conclude that lower respiratory tract afferents do not play a significant role in the perception of inspiratory resistive loads. The finding of an abnormally high Weber fraction in the heart transplant population remains unexplained but may be a function of age or restrictive pulmonary defects.