Changes in optokinetic nystagmus by age were assessed, and preparation of reference values by age was made by the use of the data from 834 reference subjects. As a result, changes by age were observed at the test on optokinetic nystagmus. The reference values by these changes can be approximated with the quadratic and was considered, therefore, to be useful clinically. Four parameters were set for the test items for optokinetic nystagmus as the total number of nystagmus (NYS); the mean of eye velocity during the fast phase of nystagmus (FM); the algebraic summations of eye velocity (VEL) and amplitude (AMP) during the slow phase of nystagmus. As the process for statistical analysis, variables were divided into 7 groups, and after removing outlier (any extraneous data) in each group, changes in variables by age were assessed by the one-way analysis of variance and the method of multiple comparison (Ryan's method). As a result: 1) the age group of each variable seemed to develop into normal distribution; 2) it was presumed that change in NYS by age is less; 3-1) regarding FM, VEL and AMP, no change was observed in the variable by ages on 30s and 40s; and 3-2) however, decrease in the measured value was observed in ages on 60s along with aging. From these, the ages on 50s are the generation when the measured values of optokinetic nystagmus change. On the other hand, the reference values calculated, taking age into consideration, could be approximated with the quadratic.(ABSTRACT TRUNCATED AT 250 WORDS)