Physiological psychology is the study of interactions; an exam ination of the way natural fluctuations within the brain and body (‘under the skin’ phenomena) lead to alterations in be haviour (actions, thoughts, emotions, etc.); and a correspond ing analysis of the way the body responds to natural or imposed psychological situations such as perceptual problems, learning tasks, emotional conflicts, and others. As will be seen shortly investigations of this two-way process may be subsumed, for convenience, within two distinct but complementary strategies each of which focuses upon information flowing in one partic ular direction in the two-way transaction. However, it must be remembered that the interaction between physiological and psychological events is dynamic and everchanging. The work ing of the total system leads to the display of integrated sequences of activities which depend upon the continual feed back of information between the physiological and psychologi cal environments. The intimate interdependence of these en vironments means that the goal of physiological psychology must ultimately involve an exploration of the fine structure of this subtle and detailed network of reciprocating relationships.