Overnight light exposure of cultured human peripheral blood mononuclear leukocytes [PBML], significantly increased basal [3H]thymidine incorporation and upon stimulation with phytohemagglutinin [PHA]. Melatonin (10(-9) to 10(-5) M) enhanced the light-induced increase of [3H]thymidine incorporation, while serotonin (10(-9) to 10(-7) M) stimulated [3H]thymidine incorporation in the dark. The wavelengths responsible of this effect were restricted to the blue-green zone of the spectrum. The stimulatory effect of visible light on PHA-induced DNA replication had a circannual rhythm, being maximal during winter. In winter, white light also reduced melatonin and serotonin binding to PBML membranes and switched the PBML indole metabolism towards serotonin and 5-hydroxy-indole-acetic acid [HIAA] synthesis, with a concomitant decrease of melatonin production.