Three separate groups of conditional reflex studies were presented to illustrate their implication for the general problem of reaction to placebo. The first demonstrates how the effect of a person can be conditioned and thereby makes it possible for the expectations of patients from a therapeutic situation to be realized. The second emphasizes the importance of "central excitatory states" for conditioning and suggests that the ability of doctors to produce these may make patients amenable to their doctors' expectations of them. The third describes the pertinence of the state of the organism and the principles of extinction to patient responses to placebo. It is suggested that placebo effects may be more explicable when considered in terms of learning concepts.Abstract Teaser