The concept of behavioural toxicity is discussed historically and traced from its recent origins in psychopharmacology. It is about adverse psychological effects of chemicals absorbed into the body. It became a significant issue in connection with the new 'psychotropic' drugs of the 1950s and has remained so. But the 'mad hatter' poisoned by mercury has been known for much longer. Changes in behaviour can be among the most sensitive indicators of toxic effects. Investigations of behavioural toxicity are considered together with their implications for further improvement of behavioural methods for the early detection of toxic effects of pollutants.