Mescaline chloride (in solutions up to 33 per cent by weight) was applied topically to the cortex and lateral geniculate of the cat and its effect on evoked responses was studied. Responses were evoked by local cortical stimulation and from appropriate sensory receiving areas by flash stimuli to the eye, both of white and of four colors; click to the ear; and electrical stimulation to the sciatic nerve. With all types of stimuli mescaline had the same marked effect on amplitude and latency of evoked cortical potentials. The effect appeared within 5 min., reached its maximum at about 30 min. and declined after 2 to 4 hours. The amplitude was increased up to 30 times and this was attributable to an increase in the amplitude of a second component of the response. The first component increased two- to threefold or might even be slightly decreased. The maximal response was obtained with stimuli of all intensities and in the visual system with light of all colors. The latency was prolonged to light flash and to direct cortical stimulation. The latency to sciatic and to acoustic stimulation was not changed. Responses were evoked only by adequate stimuli; no responses were evoked from visual cortex by other than visual stimuli, and visual stimuli evoked responses only from known optic receiving areas. When the cortical effect was fully developed, large (2 mV.) spontaneous spikes appeared. At lateral geniculate, mescaline produced a two- to fourfold increase in all components of the response to light flash. Latency was not altered. Local strychnine (3.5 per cent) produced qualitatively the same effect as mescaline on evoked responses to light flash but the effect was more marked in that the amplitude of the response was greater and the full effect developed more quickly. In other respects—increase in latency, increase in amplitude of a second component of the response, “all or none” response to every stimulus and evokation of spontaneous spikes — strychnine and mescaline acted identically on evoked responses to light flash. Intravenous mescaline in doses up to 15 mg. per kg. produced immediate, temporary autonomic changes consisting of cardiac irregularity, periods of apnoea, piloerection, urination and defecation. There was a slight decrease in the amplitude and increase in the duration of cortical and geniculate potentials evoked by light flash. Topical and intravenous lysergic acid diethylamid (up to 9 per cent solution by weight, or 40 μg. per kg.) produced no clear cut effect comparable to that of mescaline on evoked response amplitude. The time interval between the first and second components of the cortical response was prolonged.