A controlled, clinical trial, comparing cyaninoside chloride and Heleniene , was conducted on 31 out-patients suffering from functional disturbances of vision in low-luminance conditions. The evolution of photopic and mesopic visual acuities, electro- oculograms and adapto -electroretinograms was assessed for both treatment groups and controls. Both agents significantly improved photopic visual acuity (p less than 0.05). Only cyaninoside chloride treatment improved visual functions related to mesopic and scotopic vision (p less than 0.01). There were also significant differences between the two treatment groups regarding the velocity of visual adaptation in adapto -electroretinography. This study thus demonstrates the therapeutic value of cyaninoside chloride for the treatment of functional disturbances of mesopic and scotopic vision, especially in night blindness and myopia.