The present study investigated potential antidepressant effect of light exposure in the dark phase of a 12:12 L/D cycle on behavioral despair. In Exp.1, male Wistar rats were administered a single, 10 min broadband light pulse (1300lx) either 3h (ZT15) or 9h (ZT21) after dark onset (ZT12) and tested in two consecutive swim tests separated by 24 h. Photic stimulation at ZT21 but not ZT15 significantly reduced immobility in the second swim test relative to the first test compared to controls that were treated similarly except for light pulse administration (p<0.05). In Exp.2, groups were exposed to a single 10-min light pulse (1300lx) either in the blue or red end of the spectrum at ZT21 or were treated similarly except for photic exposure (controls). Exposure to blue light resulted in significantly reduced duration of immobility in the second swim test relative to that of the first test compared to the red light and control groups (p<0.05). The present findings suggest a critical role in the antidepressant effect of blue light stimulation for the melanopsin-containing ganglion cells in the retina that are sensitive to wavelengths in the blue but not red end of the visible spectrum.