A single type of olfactory sensilla on maxillary palps in many species of mosquitoes houses a very sensitive olfactory receptor neuron (ORN) for carbon dioxide reception. Extensive single sensillum recordings from this peg sensillum in Culex quinquefasciatus, an important West Nile vector, were performed to characterize the response threshold and kinetics for CO2 reception. An ORN detects CO2 with a threshold less than the atmosphere concentration. This ORN responded in a tonic mode to lower concentrations of CO2, whereas higher concentrations generated a phasic-tonic mode of action potential firing. Sensillum potentials accurately represented the response magnitude and kinetics of carbon dioxide–elicited excitatory responses. Stimulation of these ORNs with human breath, a complex mixture of mosquito kairomones and up to 4.5% CO2, elicited excitatory responses that were reliably detected by CO2-sensitive ORNs. We further evaluated the effect of various background concentration of CO2 on absolute detection. Another ORN housed in these sensilla responded to 1-octen-3-ol and to various plant-derived compounds, particularly floral and green leaf volatiles. This ORN showed remarkable sensitivity to the natural enantiomer, (R)-(─)-1-octen- 3-ol, rivaling pheromone-detecting ORNs in moths. Maximum neuronal response was elicited with a 10 ng dose. A biological, ecological role of maxillary palps in detection of plant- and nectar-related sources will be discussed.