Initial experiments were carried out to determine the dosage mortality response of the rust-red flour beetle Tribolium castaneum to two insect growth regulators, the chitin synthesis inhibitor chlorfluazuron and the juvenile hormone analogue pyriproxyfen. The response was measured as the proportion of adults developing from individuals treated during the pupal stage. For males, at the LC50 level, chlorfluazuron was more toxic than pyriproxyfen with LC50 values of 10.6 and 12.6 ppm respectively. For females, pyriproxyfen was more potent than chlorfluazuron with LC50 values of 7.1 and 8.3 ppm. Subsequent experiments were carried out using adults that had survived after treatment at the LC50 level during the pupal stage. Both responses to pheromone and the production of pheromone, by adults of both sexes that had been treated during the pupal stages with pyriproxyfen, were significantly more affected than those treated with chlorfluazuron. Both treatments caused abnormalities in the antennae of adults of both sexes.