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Previous work suggests that photoreceptor synaptic ribbon length and absolute dark-adapted threshold may vary during a 24-h diurnal cycle. To test this hypothesis, we examined the length of photoreceptor synaptic ribbons and the dark-adapted threshold in black (+/+) and albino (c2J/c2J) C57BL/6J mice at six times over a 24-h period. Testing began 2 h after light onset (ZT 2:00) and continued at successive 4-h intervals (12 h:12 h light:dark). We determined the length of the synaptic ribbons in frozen sections by labeling them with an antibody specific for synaptic ribbons. Synaptic ribbons vary in length at different points in the diurnal cycle in both types of mice, but the synaptic ribbons in black mice are longer than those in albino mice by an average of 0.33 μm. The synaptic ribbons of black mice also have a larger response to changes in the light cycle. Ribbon length in black mice ranges from 1.66 μm to 1.4 μm …