To distinguish optic nerve conduction defects from macular disease in patients with otherwise unexplained loss of central vision we first determined the best visual acuity with correction at distance in unilateral defects. The normal eye was tested first and photostressed for ten seconds by looking at an ordinary penlight held 2 to 3 cm from the eye. The time required to read three letters on three Snellen test lines just larger than the best acuity was used as the end point. In 63 eyes with maculopathy the recovery time was prolonged. Prolonged recovery time was not observed in 20 patients who had optic nerve disease.