This study measured the effects of superior colliculus (SC) lesions on skill at localizing stationary or moving objects, and on ability to discriminate between objects which differed in pattern or brightness in squirrel monkeys. Bilateral electrolytic lesions of the SC were made in 8 monkeys (EXP group); the lesions in 5 (SC group) were restricted essentially to the SC (22-25% damaged), the other 3 had additional unilateral damage to the midbrain reticular formation (10 to 14% damage to SC). The control group (N = 15) consisted of 9 unoperated, 3 sham-operated, and 3 lesioned-control monkeys. General cage behavior of the EXP group was not significantly changed as a result of the lesions. Eye movements were observed in all monkeys with SC lesions. Pupillary constrictions to light were also present except for a temporary unilateral mydriasis (two cases). On tests which measured reaction time, localization ability, and brightness discrimination, performance between the SC and control groups did not differ significantly. The SC group compared with controls, however, required more total trials to learn to criterion the pattern discrimination (p less than .05), and made significantly fewer correct choices (p less than .02) at the middle delay interval of the delayed response test. The two groups did not differ significantly at the shorter or longer delays. With regard to the possible neural mechanism, it is suggested that the SC of primates may influence level of arousal and contribute to ability to discriminate visual detail.