['To examine whether the diameters of retinal branch vessels of the human eye change during dark and light adaptation.', 'Images (S-VHS recordings) were obtained of the peripapillary region in 11 eyes of 11 healthy young adults (seven women, four men; mean age, 26.4 years). The images were made under a sequence of different illumination conditions (light, 30 minutes of darkness, light) with a scanning laser ophthalmoscope (SLO), using near-infrared illumination (785 nm). The recordings were then analyzed with a retinal vessel analyzer (RVA), and the caliber changes of one branch artery and one vein were measured in each eye.', 'For arteries, the changes of diameter under different illumination conditions showed no clear trend, and comparisons between the different time sections revealed no statistically significant changes (P = 0.933; repeated measures ANOVA). There was a slight dilation (average, 0.9%; range, -3.9% to +5.1%) in darkness, and a return to baseline (range, -2.9% to + 2.9%) on restoring normal illumination. Veins during darkness showed a small but fairly consistent constriction (average, 1.5%; range -5.4% to +3.9%; significant P = 0.05), again returning to baseline (range, -2.1% to +2.6%) in normal light.'] Do the diameters of the human retinal branch vessels change in darkness?