Does transgenically incorporated insect resistance affect constitutive and herbivore-inducible terpenoid emissions and multitrophic communication under elevated atmospheric CO(2) or ozone (O(3))? This study aimed to clarify the possible interactions between allocation to direct defences (Bacillus thuringiensis (Bt) toxin production) and that to endogenous indirect defences under future climatic conditions. Terpenoid emissions were measured from vegetative-stage non-Bt and Bt Brassica napus grown in growth chambers under control or doubled CO(2), and control (filtered air) or 100 ppb O(3). The olfactometric orientation of Cotesia vestalis, an endoparasitoid of the herbivorous diamondback moth (Plutella xylostella), was assessed under the corresponding CO(2) and O(3) concentrations. The response of terpenoid emission to CO(2) or O(3) elevations was equivalent for Bt and non-Bt plants, but lower target herbivory reduced herbivore-inducible emissions from Bt plants. Elevated CO(2) increased emissions of most terpenoids, whereas O(3) reduced total terpenoid emissions. Cotesia vestalis orientated to host-damaged plants independent of plant type or CO(2) concentration. Under elevated O(3), host-damaged non-Bt plants attracted 75% of the parasitoids, but only 36.8% of parasitoids orientated to host-damaged Bt plants. Elevated O(3) has the potential to perturb specialized food-web communication in Bt crops.