The aggressiveness of Myrmica rubra workers is stimulated by their poison gland secretion. On the other hand, the major compound of the mandibular gland, 3-octanone, markedly suppresses their aggressiveness. The inhibitory activity of 3-octanone is still effective when dissolved in liquid paraffin at a concentration of 10 −9 vol./vol. At a concentration 100 times greater in liquid paraffin, 3-nonanone may also suppress the aggressiveness of the ants. This substance however represents only a trace of the mandibular secretion. Neither 6-methyl-3-octanone, nor 3-octanol, possesses an inhibitory effect on the aggressiveness of the ants even when present in liquid paraffin at a concentration 10 4 times greater than the lowest effective dilution of 3-octanone. The relative proportions of 3-octanol and 3-octanone vary greatly in the mandibular gland secretion of the workers from the same nest or from different nests of the same species. The proportions of 3-octanol in the mixture of octanol+octanone present in the vapour released by one crushed worker head range from 4·6 to 38·0 per cent. The histogram of these proportions is unimodal, with the mode situated near 15 per cent of 3-octanol. A mixture of 3-octanol and 3-octanone in liquid paraffin, releasing a vapour containing 15 per cent of 3-octanol, has no more activity than a solution containing only 3-octanone. There is no reason to believe so far that a mixture of 3-octanol and 3-octanone of precise composition has any particular meaning for the ants. We suggest that the ants when attacked emit the inhibitory and stimulative pheromones in such a way that the aggressiveness of the alarmed workers is directed toward the enemy without their being themselves the victims of this aggressiveness.