A thin slit-lamp beam illuminating the pupil margin produces clearly visible pupil oscillations. These oscillations can be timed with a stopwatch, thus producing a measurement of the "edge-light" pupil cycle time'. The pupil cycle time is remarkably stable in various testing situations and is repeatable within about +/- 3% over extended periods of time. Expected normal distribution and 95% confidence intervals are given in Table 3. When the iris muscles are normally innervated and responsive, the pupil cycle time is dependent on the speed of nerve conduction and the number and strength of optic nerve impulses. Only 5% of normal persons aged 12 to 50 years are expected to have a pupil cycle time in either eye longer than 954 ms or a difference in pupil cycle time between the two eyes longer than 70 ms.