Stumptail monkeys (Macaca arctoides) performed a discrete trial, three-choice visual discrimination. The discrimination behavior was controlled by the shape of the visual stimuli. Strength of the stimuli in controlling behavior was systematically related to a physical property of the stimuli, luminance. Low luminance provided weak control, resulting in a low accuracy of discrimination, a low response probability and maximal sensitivity to scopolamine (7.5--60 mug/kg). In contrast, high luminance provided strong control of behavior and attenuated the effects of scopolamine. Methylscopolamine had no effect in doses of 30 to 90 mug/kg. Scopolamine effects resembled the effects of reducing stimulus control in undrugged monkeys. Since behavior under weak control seems to be especially sensitive to drugs, manipulations of stimulus control may be particularly useful whenever determination of the minimally-effective dose is important, as in behavioral toxicology. Present results are interpreted as specific visual effects of the drug, since nonsensory factors such as base-line response rate, reinforcement schedule, training history, motor performance and motivation were controlled. Implications for state-dependent effects of drugs are discussed.