The effect of sustained psychosocial stress on the morphology of hippocampal pyramidal neurons was analysed in male tree shrews after 14, 20, and 28 days of social confrontation. A variety of physiological changes such as constantly elevated levels of urinary cortisol and norepinephrine and reduced body weight, which are indicative of chronic stress were observed in the subordinate, but not in the dominant males. Light microscopic analysis of Nissl-stained hippocampal sections showed that the staining intensity of the nucleoplasm in the CA1 and CA3 pyramidal neurons was increased after prolonged psychosocial stress, indicating a change in the nuclear chromatin structure. These alterations were observed only in subordinate animals and increased in a time dependent manner in accordance with the length of the stress period. There was, however, neither a reduction in density nor a degeneration of pyramidal neurons in chronically stressed animals. Mechanisms which may possibly account for the observed alterations are discussed.