It is well known that volatile cues from damaged plants may induce resistance in neighboring plants. Much less is known about the effects of volatile interaction between undamaged plants. In this study, barley plants, Hordeum vulgare cv. Kara, were exposed to volatiles from undamaged plants of barley cv. Alva or thistle Cirsium vulgare, and to the volatile phytochemicals, methyl salicylate or methyl jasmonate. Exposures were made either during natural daylight or darkness. Acceptance of exposed plants by the aphid Rhopalosiphum padi was assessed, as well as the expression of putative marker genes for the different treatments. Aphid acceptance of plants exposed to either barley or C. vulgare was significantly reduced, and an effect of the volatiles from undamaged plants was confirmed by the induction of pathogenesis-related protein, PR1a in exposed plants. However the effect on aphid acceptance was seen only when plants were exposed during darkness, whereas PR1a was induced only after treatment during daylight. Aphid acceptance of plants exposed to either methyl salicylate or methyl jasmonate was significantly reduced, but only when plants were exposed to the chemicals during daylight. AOS2 (allene oxide synthase) was induced by methyl jasmonate and BCI-4 (barley chemical inducible gene-4) by methyl salicylate in both daylight and darkness. It is concluded that (a) the effects on aphids of exposing barley to volatile phytochemicals was influenced by the presence or absence of light and (b) the response of barley to methyl salicylate/methyl jasmonate and to volatiles from undamaged plants differed at the gene and herbivore level.