There is no discussion in Stevens' (16) or Osgood's (14) textbooks of experimental psychology of the contribution of cortical functions to the production of visual after-images. Woodworth (19) presents a theory of after-images involving the cortex in the first edition of his book. It is not repeated in the second edition. There is, however, a large number of reported studies indicating the importance of cortical functions in the production of after-images (e.g. 1, 3, 6, 9, 11, 12, 13, 15, 17). In view of these studies it was decided to use the afterimage in a further attempt to verify or disprove the drug action hypothesis advanced by Eysenck in a previous paper (2). This hypothesis stated that stimulant drugs increased excitatory potential and decreased inhibitory potential, while depressant drugs decreased excitatory potential and increased inhibitory potential.