Mescaline produces altered visual experiences of several types, whose vividness and degree of distortion is proportionate to the size of the dose. There are no sharp boundaries dividing abnormal sensibility, illusions, and hallucinations. Subjects who gave fewer responses of the type "I see it" when asked to imagine situations were more likely to hallucinate than other patients. Visual responses declined still further after administration of mescaline in a degree more closely related to the amount of drug given than to the vividness of hallucinatory alterations, even in those experimental subjects who reported virtually no alteration in actual vision. The fall in reported visual imagery was not found in control subjects.