BACKGROUND:The existence of a diurnal variation in the Intra ocular Pressure (IOP) of both normal and glaucomatous eyes is well documented. However, the magnitude and pattern of that variation remains in conjecture. AIM: To study the diurnal fluctuation of the IOP in the normal subjects. MATERIALS AND METHODS: The study group consisted of 92 volunteers, visually normal as subjects, 50 boys and 42 girls in the age group of 18 to 20 years. Independent measurements of the IOP of each eye were obtained during the day with the subject in the upright position. The first measurement was undertaken between 8-9AM, the second between 12 Noon-1PM and the final reading between 5-6PM. The subjects carried normal routine activity during the period of measurement. Keelar Pulsair air impulse tonometer was used in all the subjects for IOP measurement. IOP was measured in the department of Ophthalmology, Teaching Hospital. STATISTICAL ANALYSIS: were done using Paired 't' test. RESULTS: The mean IOP of all the subjects decreased by 2.304 mmHg during the day. The change was statistically significant (<0.001). Mean IOP decreased by 2.977mmHg during the day in 83.69% of the subjects, all showing peak pressures in the morning. In a small proportion of subjects (10.86%) the mean increased by 1.75mm Hg which was not statistically significant. In 5.43% of the subjects the IOP remained the same during the day. The present study did not find any association between age, sex and the IOP parameters examined (Time of peak, trough and diurnal fluctuation). CONCLUSION: IOP decreased during the in a large number of subjects (83.69%). The peak IOP was before noon. The diurnal IOP variation is quite minimal averaging 3-4 mm Hg but a single measurement taken during the late afternoon or evening may miss IOP peak. There is no method to continually measure IOP. Even a large number of pressure measurements dispersed through a 24-hour period may miss the peak pressure for that day. Thus, it is obvious that any choice of measurement is arbitrary. Many factors are responsible for daily fluctuations in IOP. Some of these factors operate at the level of the eye.