The author reviews the instructions given to the subjects of a variety of sensory deprivation experiments (Hebb, Lilly, etc.). As a result of his work with Ruff and Thaler at the Aero-Space Medical Laboratories from 1957 to 1959, he reports that their findings were at variance with other workers in that they obtained few such striking regressive responses as hallucinations, disorganized thinking, or severe anxiety. He suggests that the differences are accounted for by the nature of instructions given to the subject, how much the subjects know