Abstract: Although there have been many reports that conditioned immunosuppression (CIS) can be achieved through behavioral conditioning techniques, little is known about the neural mechanisms of CIS. In this study, cyclophosphamide (CY) was used as an unconditioned stimulus (UCS) with saccharin as the conditioned stimulus (CS) in a conditioned taste aversion (CTA) paradigm. Both CTA and CIS as assessed by peripheral leukocyte and lymphocyte were examined by one re-exposure to CS at intervals of 5, 30 days after two trials of CS-UCS pairing. The effect of one re-exposure to CS on the expression of FOS-protein in the hypothalamus nucleus was also observed correspondingly. The results showed that CTA occurred on both day 5 and day 30 after conditioning but CIS occurred only on day 5 after conditioning. It was further found that some nuclei of hypothalamus, paraventricular nucleus (PVN) in particular, showed high expression of FOS-protein on the fifth day after one re-exposure to CS, but there was no or little expression of FOS-protein on the thirtieth day. By comparing the results of the fifth day with the thirtieth day，it is suggested that PVN of the hypothalamus is an important nuclear for modulating the response of CIS in the CNS.

Key words: conditioned immunosuppression, hypothalamus nuclei, c-fos