Objective: To study the effects of light/dark adaptation times on flash electroretinograms in normal adult  cynomolgus monkeys. Methods: In this experimental study, three adult cynomolgus monkeys were  selected and stimulated with standard electroretinogram (ERG) parameters after dark adaptation for 20 min,  40 min and 60 min. The scotopic-ERG was recorded using aroutine procedure. After dark adaptation for  40 min, the photopic-ERG was recorded for 1 min, 5 min and 10 min. One-way ANOVA was used to compare  the amplitude and latency. Results: There was no significant difference in the latencies of a and b waves  and OPs in the scotopic-ERG (F=0.052, P=0.949). When the dark-adaption time was less than 40 min, the  amplitudes of the a and b waves and OPs increased significantly as the adaptation time increased (F=50.800,  P<0.001). When the dark-adaptation time was more than 40 min, the amplitude reached the maximum and  no longer increased with time (F=0.016, P=0.899). For the photopic-ERG, the implicit times of the a and b waves were also unaffected by the length of the light-adaptation time (F=0.980, P=0.381). The amplitudes  of the a and b waves reached their maximum after 5 minutes of light adaptation (F=4.789, P=0.036) and  tended to be stable over time (F=0.135, P=0.717). Conclusions: In a certain period of time, the results of  flash ERG of cynomolgus monkeys are greatly affected by bright/dark time. So, the dark-adaptation time  for scotopic-ERG recording in cynomolgus monkeys should be at least 40 minutes, and the photopic-ERG  are best recorded after 5 minutes of light adaption.