ABSTRACT. Stress-induced hyperthermia is observed in animals exposed to stressful conditions. In our previous study, plant-derived fragrances such as green odor and alpha-pinene were shown to suppress this stress response in rats. In the present study, we examined the concentration-dependence of the alpha-pinene effects on stress-induced hyperthermia. Male rats carrying telemetry transmitters were transferred individually to a new cage containing bedding that had been sprayed with 0.3, 0.03, and 0.003% concentrations of alphapinene or control solvent. Following transfer to the novel environment, the body temperature increased significantly, and this response was clearly suppressed when the cage was scattered with 0.03% alpha-pinene only. These results suggest that the effect of alpha -pinene on stress-induced hyperthermia can be observed only at a certain concentration. KEY WORDS: alpha-pinene, concentration effect, hyperthermia.J. Vet. Med. Sci. 65(9): 1023-1025, 2003 When an animal is exposed to stressful situations, autonomic responses including a temporary increase of body temperature, namely stress-induced hyperthermia or emotional fever, have been observed in several species [3,4,[6][7][8][9][24][25][26]. Recent studies have suggested that in phylogeny this stress response involving body temperature has emerged between amphibians and reptiles [5]. In our previous study, it was revealed that an external olfactory stimulation could affect this autonomic stress response. Namely, plant-derived fragrances such as green odor and alphapinene showed ameliorative effects on stress-induced hyperthermia [1]. In addition, by using this model Kikusui et al. could also show an opposite effect in which exposure to alarm pheromones resulted in an aggravation of this hyperthermia [13]. When the odorant concentration was changed, the impression or the effects accompanying that odor to the physiological response and behavior in vivo could be different. For example, the event-related potential P300 was altered its amplitude by emotional change, and the experiment using components of the green odor induced different amplitudes of P300 depending on its concentration [20]. Feeding behavior of goats was suppressed by only higher concentration of trans-2-hexenal [11]. These results suggest that each odorant has a certain concentration at which it is most effective. In the present study we therefore examined concentration-dependence of the effect of alphapinene on stress-induced hyperthermia by preparing both lower (0.003%) and higher (0.3%) concentrations in addition to the concentration (0.03%) that in our previous study was confirmed to be effective.The basic experimental procedures and animals were as described previously [1]. Briefly, male Wistar rats (9 weeks old: Clea Japan, Inc., Tokyo, Japan) weighing approximately 230-300 g were housed in groups of 4 animals per cage with wood shavings (Soft chip: Japan SLC, Inc