Histamine aerosols were administered to nine anesthetized, paralyzed, mechanically ventilated mongrel dogs with the cervical vagi first intact, then sectioned, and then peripherally stimulated at two intensities. Pulmonary resistance (RL) was measured, and dose-response curves were constructed in the four conditions. All dogs had dose-related increases in RL with increasing histamine aerosol concentrations. After the vagi were cut, the degree to which constant levels of vagal stimulation altered the dose-response relationships was assessed by examining the slopes and positions of the RL vs. histamine curves. Four of the nine animals studied showed evidence of increasing vagal efferent activity as the concentration of histamine in the aerosol increased, but three did not. Two of the dogs had equivocal responses in that increasing activity was suggested but not clearly demonstrated. These results indicate that, in addition to direct effects of histamine, the extent of and basis for vagal interaction with this stimulus varies among dogs. The results suggest that the basis of vagal interaction is related to either constant efferent activity (tone) or increasing efferent activity (reflexes).