It is concluded that measurement of visual readaptation time (RAT) using optokinetic nystagmus (OKN) is a repeatable measure of visual recovery after flash exposure. The semi-automatic method for measurement of RAT used here requires further development, but it is anticipated that the improved method will provide an efficient tool for increased understanding of the physiology of flash blindness. In this study on humans, it was found that if RAT is recorded twice on the same occasion, the second RAT is shorter. However, there was no systematic difference between RAT recordings on consecutive occasions. The newly developed semi-automatic method was found to provide RATs comparable to those obtained by manual measurement on a paper print out of EOG recordings. In RAT estimation, the variability between subjects shadows other sources of random variability. The least number of subjects needed in each group to detect a 20% alteration of RAT due to an experimental factor (alpha = 0.05, beta = 0.05) was estimated to 13 with independent groups design. For paired design < 10 are needed. OKN was elicited with a horizontally moving vertical grating. The eye movement was recorded by DC EOG. A sudden flash of green light temporarily abolished the OKN. The internal between the flash and the reappearance of OKN was measured as the RAT.