In previous electrophysiological experiments from hypopigmented animals (mice, rats, rabbits), single-unit recordings from both retinal ganglion axons and cells in the superior colliculus have demonstrated an increase in threshold in the dark-adapted state which is roughly proportional to the ocular melanin concentration. In the present study we compared an albino mouse strain which is relatively resistant to light damage and the beige mouse mutant to their wild-type controls in a situation that involved unanesthetized, unrestrained mice as a control to the electrophysiological single unit experiments. We used a six-chambered water maze. Animals were trained to swim to an illuminated ramp until their performances leveled off (about 10 days). The animals were then dark-adapted for 24 h and tested after reducing the luminance level of the water maze. We found that the albino mice failed to find the ramp when the luminance fell to 1.58 x 10(-3) cd/m2 (p < or = .0001), the beige mice failed at 2.00 x 10(-4) cd/m2 (p < or = .0001), and the normally pigmented controls performed to 5.00 x 10(-5) cd/m2 (p < or = .0001). These results support our previous findings that the sensitivity defect in hypopigmented animals is proportional to the degree of ocular hypopigmentation.