The influence of the antipsychotic agents on medical practice has been compared to the impact of the antibiotics in terms of numbers of patients affected, quantity of research effort generated, and changes in patterns of care introduced. Are the almost routinely encountered neurologic (neuroleptic) effects of the antipsychotic drugs essential to their actions? The mechanisms that have been proposed to explain the actions of these drugs are examined here. The side effects are discussed in part 2, which begins on page 123.