PURPOSE. Recent epidemiologic studies have shown that children who spend a higher proportion of time outdoors are less likely to develop myopia. This study was undertaken to investigate whether light levels may be a relevant factor in the development of myopia. METHODS. Paradigm 1: Chicks were fitted with translucent diffusers for 5 days, with the diffusers removed daily for 15 minutes under one of three lighting conditions: (1) normal laboratory lighting (500 lux), (2) intense laboratory lighting (15,000 lux), or (3) daylight (30,000 lux). A control group, which continuously wore diffusers, was also kept under an illumination of 500 lux. Paradigm 2: Chicks fitted with translucent diffusers were raised for 4 days under one of three lighting conditions: (1) low laboratory lighting (50 lux, n = 9), (2) normal laboratory lighting (500 lux, n = 18), or (3) intense laboratory lights (15,000 lux, n = 9). In groups 1 and 3, the chicks were exposed to either low or high ambient illuminances for a period of 6 hours per day (10 AM-4 PM), but were kept under 500 lux for the remaining time of the light phase. Axial length and refraction were measured at the commencement and cessation of all treatments, with corneal curvature measured additionally in paradigm 2. RESULTS. Paradigm 1: The chicks exposed daily to sunlight for 15 minutes had significantly shorter eyes (8.81 ± 0.05 mm; P < 0.01) and less myopic refractions (-1.1 ± 0.45 D; P < 0.01) than did the chicks that had their diffusers removed under normal laboratory light levels (8.98 ±0.03 mm, -5.3 ± 0.5 D). If the diffusers were removed under intense laboratory lights, the chicks also developed shorter eyes (-8.88 ± 0.04 mm; P < 0.01) and less myopic refractions (-3.4 ± 0.6D; P < 0.01). Paradigm 2: The chicks that wore diffusers continuously under high illuminance had shorter eyes (8.54 ± 0.02 mm; P < 0.01) and less myopic refractions (+0.04 ± 0.7D; P< 0.001) compared with those chicks reared under normal light levels (8.64 ± 0.06 mm, -5.3 ± 0.9 D). Low illuminance (50 lux) did not further increase deprivation myopia. CONCLUSIONS. Exposing chicks to high illuminances, either sunlight or intense laboratory lights, retards the development of experimental myopia. These results, in conjunction with recent epidemiologic findings, suggest that daily exposure to high light levels may have a protective effect against the development of school-age myopia in children.