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The results of the present investigation differ slightly from the findings of the earlier study. In the earlier investigation both the ataractic, meprobamate, and the hypnotic, glutethimide, reduced the extent of the inner threshold of the visual field compared with the control, no-drug, condition. In this present study neither of the drug treatments changed the inner threshold sufficiently to produce a reliable difference. The outer threshold changes, on the other hand, are consistent with earlier findings, namely, the depressant drug Amytal has extended the visual field compared to the Placebo treatment. Again, as in the earlier study, the difference between the inner and outer threshold (the interval of uncertainty) demonstrates a profound change, attributable to the drugs. Finally, when the increase in the interval between thresholds is examined in relation to increasing target intensity, drug effects also emerge which indicate that Amytal enlarges the interval whereas Dexedrine narrows it. The finding of an interval of uncertainty drug effect implies one of two possible explanations : ( a ) a slowing of motor response in reacting to decreases in stimulus intensity; or ( b ) some change in the sensory scale of discrimination of brightness differences ; both explanations being based upon the central action of the drugs. The first of these explanations was considered after the first study but rejected owing to its incompatibility with other test findings which required rapid responses. However, other authors who have been concerned with visual field reductions, under conditions of anoxia (Ikui, 1947), do not reject the possibility ofgeneral response slowing. The second explanation would involve an investigation of the psychological changes which may follow drug ingestion. Both explanations are, of course, questioned due to the finding that the drug effects only appear to manifest themselves to a decreasing intensity, i.e. an outgoing stimulus. Finally, the present method of measurement does not permit discrimination between explanations as, clearly, the two factors of discrimination and response are confounded. An alternative method is called for in which brightness discrimination is not influenced by speed of response. Such measurements are available in the constant methods and future experiments will be attempted by their use. If drug effects can be demonstrated as inducing changes in psychosensory continua, integration into general psychological theory seems possible.