Light has been shown to positively affect mood in a population of patients who get depressed in the winter. For light to have a positive response, the patient must be exposed to bright light (3000lux) for a few hours each day. During this past year we sampled patients "on" or "off" light therapy to assess peripheral blood lymphocyte activity during these two conditions. Curiously, lymphocyte activity was significantly suppressed when they were "on" light treatment, and showed an improvement in mood. Based on these observations we exposed rats to the same bank of light for a week. Splenic lymphocyte activity was markedly elevated following this chronic exposure to light. However, 4-h exposure to light suppressed lymphocyte activity. These data suggest that light may directly or indirectly affect the immune system.