Isolation-rearing of rats causes a variety of behavioral changes, including anxiety, learning deficits and sensory changes related to schizophrenia. Similar changes are seen following loss of serotonin during development. Thus, the effects of isolation-rearing on behavior may be due to changes in serotonin. Sprague-Dawley rats were raised in groups of four (social animals) or in isolation, from postnatal day 22 until postnatal day 64. The hippocampi were examined immunochemically for changes in serotonin. Our findings show that serotonin terminals are lost throughout the CA regions of hippocampus, where there is also an associated loss of dendrites, but not in the molecular layer of the dentate gyrus. Thus, some of the brain and behavioral changes seen in isolation-reared animals could be due to loss of serotonin.