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 microm. 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 microm to 1.4 microm, whereas ribbon length in albino mice ranges from 1.32 microm to 1.25 microm. The shortest ribbons are evident 6 h after light onset in both types of mice, whereas the longest ribbons appear within 2 h after light onset. These changes in synaptic ribbon length support the idea that photoreceptor synaptic ribbons are dynamic structures whose length changes over a 24-h diurnal cycle. Examining black and albino mice with a water-maze behavioral assay showed that visual thresholds in black and albino mice vary over the 24-h diurnal cycle. The visual thresholds of albino mice are elevated compared with black mice at all times tested. This is consistent with previous findings of visual thresholds in hypopigmented mice. The lowest threshold (greatest sensitivity) is present 2 h after light onset (ZT 2:00) and corresponds to the time when synaptic ribbons are the longest. The highest threshold is observed 6 h after light onset, the time when synaptic ribbons are shortest. These results show that synaptic ribbon length and visual sensitivity vary together in relation to the time.