Multidrug resistance-associated protein 1 (MRP1/ABCC1) is a highly abundant efflux transporter in the lungs, which protects cells from toxins and oxidative stress and has been implicated in the pathophysiology of chronic obstructive pulmonary disease and cystic fibrosis. There is evidence from in vitro studies that the inhaled glucocorticoid budesonide can inhibit MRP1 activity. We used positron emission tomography (PET) imaging with 6-bromo-7-[11C]methylpurine ([11C]BMP), which is transformed in vivo into a radiolabeled MRP1 substrate, to assess whether intratracheally (i.t.) aerosolized budesonide affects pulmonary MRP1 activity in rats. Three groups of rats (n = 5–6 each) underwent dynamic PET scans of the lungs after i.t. aerosolization of either [11C]BMP alone, or [11C]BMP mixed with either budesonide (0.04 mg, corresponding to the maximum soluble dose) or the model MRP1 inhibitor MK571 (2 mg). From PET-measured radioactivity concentration-time curves, the rate constant describing radioactivity elimination from the right lung (kE,lung) and the area under the curve (AUClung) were calculated from 0 to 5 min after start of the PET scan as measures of pulmonary MRP1 activity. Co-administration of MK571 resulted in a pronounced decrease in kE,lung (25-fold, p < 0.0001) and an increase in AUClung (5.3-fold, p < 0.0001) when compared with vehicle-treated animals. In contrast, in budesonide-treated animals kE,lung and AUClung were not significantly different from the vehicle group. Our results show that i.t. aerosolized budesonide at an approximately 5 times higher dose than the maximum clinical dose leads to no change in pulmonary MRP1 activity, suggesting a lack of an effect of inhaled budesonide treatment on the MRP1-mediated cellular detoxifying capacity of the lungs. However, the strong effect observed for MK571 raises the possibility for the occurrence of transporter-mediated drug-drug interactions at the pulmonary epithelium with inhaled medicines.