1. Bronchial responsiveness to inhaled methacholine was studied in ten heart-lung transplant (HLT) recipients a mean 10.6 months (range 1.5-28 months) post-HLT and in ten normal subjects. 2. The mean provocation dose of methacholine producing a 20% fall in FEV1 (PD20 FEV1) was significantly lower in the HLT recipients (1.70 +/- 3.96 mg vs 11.55 +/- 3.79 mg; P less than 0.001), as was the mean provocation dose of methacholine producing a 50% fall in specific airway conductance (PD50 sGAW) (0.08 +/- 0.08 mg vs 5.13 +/- 5.47 mg; P less than 0.005). 3. These results establish the presence of significant bronchial hyper-responsiveness to inhaled cholinergic agonists in the transplanted lung.