Neuroendocrine-immune interactions are thought to be important in determining susceptibility to autoimmune disease. Animal studies have revealed that differences in susceptibility to experimental autoimmune encephalomyelitis (EAE) are related to:reactivity in the hypothalamo-pituitary-adrenal axis. It is known that there is a close relation between neuroendocrine parameters and behavioral characteristics, suggesting that behavior and disease susceptibility may be associated. In the present study we investigated whether behavioral characteristics of wild-type rats are related to susceptibility to disease. We show here that the latency of the animal to attack an intruder correlates significantly with the EAE disease score: animals that do not attack the intruder during the test period are more resistant to the disease than animals with short attack latency times. These data, obtained in an unselected strain of wild-type rats, demonstrate that behavioral response patterns of individual animals can in part predict susceptibility to autoimmune disease. (C) 1999 Academic Press.