Design of inhalable mRNA therapeutics is promising, because local administration in the respiratory tract is minimally invasive and induces a local response. However, several challenges related to administration via inhalation and respiratory tract barriers have so far prevented the progress of inhaled mRNA therapeutics. Here, we investigated factors of importance for lipid nanoparticle (LNP)-mediated delivery of mRNA to the respiratory tract. We hypothesized that: (i) the PEG-lipid content is important for providing colloidal stability during aerosolization and for mucosal delivery, (ii) the PEG-lipid contentinfluences mRNA expression in the lungs, and (iii) the route of administration (nasal versus pulmonary) influences mRNA expression in the lungs. In this study, we aimed to optimize the PEG-lipid content for mucosal delivery and to investigatethe effect of administration route on the kinetics of mRNA expression. Our results show that increasing the PEG-lipid content improves the colloidal stability during the aerosolization process, but has a negative impact on the transfection efficiencyin vitro. The kinetics of protein expressionin vivois dependent on the route of administration, and we found that pulmonaryadministration of mRNA-LNPs to mice results inlonger protein expression than nasaladministration. These results demonstrate that the design of the delivery system and the route of administration are importantfor achieving high mRNA transfection efficiency in the respiratory tract.