Idiosyncratic reactions occur in a small fraction (typically Ͻ5%) of the population taking therapeutic drugs. Chlorpromazine (CPZ) is a phenothiazine, antipsychotic drug that has caused several idiosyncratic responses during its therapeutic use. Clinical evidence suggests that conditions associated with inflammation are risk factors for the appearance of these responses. Accordingly, we tested the hypothesis that an inflammatory stimulus, bacterial lipopolysaccharide (LPS), renders animals susceptible to CPZ-induced idiosyncratic reactions seen in humans. Male Sprague-Dawley rats (200 -250 g) were fasted for 24 h. A small dose of LPS (7.4 ϫ 10 6 EU/kg from Escherichia coli) or its vehicle (saline) was administered by tail vein 2 h before an intraperitoneal injection of CPZ (70 mg/kg) or its vehicle (saline). Cholestasis and hepatocellular necrosis were evaluated as increased concentrations of serum bile acids and bilirubin and increased activities of alkaline phosphatase, ␥-glutamyltransferase, alanine aminotransferase, and aspartate aminotransferase. With the exception of bile acids, these serum markers were elevated in animals treated with LPS/CPZ. Histopathological lesions in liver sections were consistent with these findings. Elevated serum creatine kinase activity, which is associated with human idiosyncratic responses to phenothiazines, was also found in animals treated with LPS/CPZ, but not with either LPS or CPZ alone. These results raise the possibility that concurrent, modest inflammation may underlie susceptibility of individuals to certain idiosyncratic reactions and may form the basis for an animal model with which to understand and predict drug idiosyncrasy.Drug idiosyncrasy is an untoward biological response to a therapeutic agent occurring in a small percentage of individuals. Idiosyncratic responses appear to occur independently of dose and have an inconsistent temporal relationship to the course of drug administration (Hollister, 1957). Although drug metabolism polymorphisms and drug allergy are widely presumed to underlie idiosyncratic responses, convincing evidence to support these as causes is lacking for the majority of drugs. Reproducing such responses in animals has been met with little success. Inasmuch as drug idiosyncrasy results in human suffering and considerable cost to pharmaceutical companies, animal models that are able to predict such responses in people before a drug is marketed could have great benefit.Among the many drugs that have caused idiosyncratic reactions in people are aliphatic phenothiazines. For example, chlorpromazine (CPZ) (Thorazine, 10-[3-dimethylaminopropyl]-2-chlorphenothiazine) is a tricyclic antidepressant that has been used as a sedative and antiemetic and for the management of psychotic disorders. Two types of adverse reactions result from phenothiazine usage. First, extrapyramidal side effects such as pseudoparkinsonism, dystonic reactions, and akathisia are common, dose-related side effects that likely result from the blockade of dopamine recepto...