The effects of postmortem time, age and tissue storage time on synaptic density were investigated in the prefrontal cortex of human brains. Thirty normal cases and 10 patients with a variety of neurologic diseases were studied, using quantitative immunohistochemistry with a monoclonal antibody to synaptophysin. Synaptophysin immunoreactivity was stable during the first 36 h after death, showing no significant change at 1, 8, 12, 24 and 36 h postmortem in normals. However, at 48 and 72 h postmortem, synaptophysin immunoreactivity decreased significantly. A similar pattern of postmortem changes was found in the disease group. In 14 normal cases with a wide range of ages, a significant reduction in synaptic density with age was observed while the time of tissue storage as paraffin-embedded blocks had no significant effect on the synaptophysin immunoreactivity. These results support the validity of using synaptophysin immunohistochemistry as a measure of synaptic density in human brain autopsied within 36 h of death and stored as paraffin-embedded blocks for a long time.