Infrared pupillography was used to determine physiological time parameters of the direct phasic light reflex in 101 normal subjects. These parameters include latency, contraction and redilatation times as well as total reaction time, measured in response to 4 different light stimulus intensities. A total of more than 4000 pupillary stimulations in these subjects was analyzed statistically. The study of these time parameters included determination of the mean, the standard deviation and three dimensional analysis of variance, with the following results: 1. The physiological time parameters are bilaterally symmetrical. 2. The most stable parameters with least variability in an individual subject were latency and contraction time. The redilatation time, however, showed marked intra-individual variability. 3. Variance analysis revealed a high degree of correlation among the different parameters to light stimuli of varying intensity. 4. There was a highly significant prolongation of latency and of contraction time with increasing age. 5. No sex dependent effect on physiological time parameters was found. In addition, no interaction among light stimulus intensity, age and sex could be detected. Possible applications of the method for clinical neurological examination are discussed.