Volatiles emitted from stressed plants can induce resistance in healthy neighbors. It remains unknown, however, how plants perceive volatiles and convert them into internal signals. We exposed lima bean (Phaseolus lunatus L.) to different concentrations of either of two volatiles, nonanal and methyl salicylate (MeSA), over 6 or 24 h. Plant resistance to the bacterial pathogen, Pseudomonas syringae, was increased significantly after exposure to a headspace with two concentrations of nonanal for 6 h, and the same pattern emerged after an exposure over 24 h. By contrast, exposure to a low concentration of MeSA over 6 h did not significantly reduce bacterial infections, whereas exposure to the same concentration over 24 h significantly enhanced resistance. The dose-response relation that was apparent after 6 h of MeSA exposure disappeared in the 24 h treatment, in which the three tested concentrations caused indistinguishable, high levels of resistance to P. syringae. A low concentration of a potentially resistance-enhancing volatile sufficed to cause resistance to pathogens in the receiver plant only after long exposure time. Plant-plant signaling appears to involve the accumulation of volatiles in the receiver.