The effects of alcohol and marijuana (tetrahydrocannabinol-THC) on saccades, smooth pursuit, and optokinetic nystagmus were quantitatively evaluated in 24 normal subjects using electro-oculographic recordings. Each subject was given an initial trial run and then tested three times (at weekly intervals) with either 0.0microgram THC or 100 microgram THC/kg bodyweight while at three different blood alcohol concentrations (0.0, 0.05 and 0.1%). A 2X3 factorial design was used. Saccades and smooth pursuit were induced by a dot of light moving in steps and ramps on a modified television set. Optokinetic nystagmus was induced by a cloth drum completely surrounding the subject and moving at a constant velocity of 30 degrees/s. Alcohol (0.05 and 0.1%) alone produced significant (p less than 0.05) impairment of saccade maximum velocity and reaction time, smooth pursuit velocity, and optokinetic slow-component velocity. The addition of THC caused performance to further deteriorate at each blood alcohol level but, in all but one instance, the added effect was not statistically significant ( greater than 0.05). At the THC and alcohol concentrations used in this study, the eye movement effects of alcohol over-shadowed those of marijuana.