Little is known about the regulation of constitutive compared to inducible nitric oxide synthase in transplanted lungs. Experiments were designed to measure levels of messenger RNA for ecNOS and iNOS in lung parenchyma following single lung allotransplantation under conditions of allograft rejection and infection. Single lung allotransplantation were performed in dogs. mRNA was extracted from lung parenchyma and assessed using Northern blot techniques. Animals were maintained on standard triple immunosuppressive therapy for five days. At that time, dogs were either assigned to Group I which was maintained on immunosuppressants, Group II which was maintained on immunosuppression and inoculated with 1010 colony forming units of antibiotic resistant E.coli, or Group III in which immunosuppression was withdrawn to allow acute rejection to occur. Relative densities were compared to cyclophilin mRNA for standardization. Plasma levels of nitric oxide as measured by chemiluminescence were increased comparatively with rejection and infection. Expression of mRNA for ecNOS was decreased significantly in lung parenchyma from rejecting and infected lung parenchyma. However, expression of mRNA for iNOS was induced comparably with both rejection and infection. These results suggest that changes in plasma nitric oxide may reflect induction of iNOS rather than over expression of ecNOS.