The physical mechanism by which light is absorbed in the eye and has antidepressant and energizing effects in Seasonal Affective Disorder and other forms of psychiatric major depression is of scientific interest. This study was designed to explore one specific aspect of a proposed humoral phototransduction mechanism, namely that carbon monoxide (CO) levels increase in retinal venous blood in response to bright light. Eleven mature male pigs approximately six months of age were kept for 7 days in darkness and fasted for 12 h prior to surgery. Following mild sedation, anesthesia was induced. Silstic catheters were inserted into the dorsal nasal vein through the angular vein of the eye to reach the ophthalmic sinus, from which venous blood outfiowing from the eye area was collected. The animals were exposed to 5000 lx of fluorescent-generated white light. CO levels in the blood were analyzed by gas chromatography before and after 80 min of light exposure. At baseline, mean CO levels in the retinal venous blood were 0.43 0.05 (SE) nmol/ml. After bright light, mean CO levels increased to 0.54 0.06 nmol/ml (two-tailed t-test p < 0.05). This study provides preliminary mammalian evidence that acute bright light exposure raises carbon monoxide levels in ophthalmic venous blood. (C) 2017 Elsevier B.V. All rights reserved.