Caffeine's effect on the visual system was investigated. Twelve male and twelve female subjects (aged 18 - 25 years) were measured for Critical Flicker Frequency (CFF) thresholds at 15 levels of retinal illuminance (-1.0 to 3.0 log trolands) in each of four caffeine dosage conditions (0, 200mg, 400mg, 600mg). Variables of interest included dosage, gender, left and right eye differences, and time after ingestion. Significant results were found for dosage (p=.000), gender (p=.001), and eye differences (p=.000). Interactions were found for gender and dosage (p=.000), and gender and eye differences (p=.043). Implications of these findings are discussed in terms of caffeine's effect on the Central Nervous System (CNS) and corresponding effects on the visual system. It is concluded that ingestion of caffeine causes increased sensitivity of the visual system as displayed through lower Critical Flicker Frequency thresholds. / Master of Science