Considerable evidence supports the involvement of acyl-CoA:cholesterol acyltransferase (ACAT) in the maintenance of intracellular cholesterol homeostasis. A number of recently developed ACAT inhibitors may have potential use as pharmacological agents to reduce the development of atherosclerosis. Recently, however, reports arose describing cytotoxic effects following administration of a specific ACAT inhibitor to experimental animals. In order to address the specific intracellular mechanisms involved with the cytotoxic effect, we examined the consequences of ACAT inhibition in cholesterol-enriched mouse peritoneal macrophages. Mouse peritoneal macrophages were cholesterol-enriched by incubation with acetylated low density lipoprotein and free cholesterol:phospholipid dispersions prior to the addition of an ACAT inhibitor, either Sandoz 58-035 or Pfizer CP-113,818. The adenine pool of the macrophages was radiolabeled prior to addition of the ACAT inhibitors, in order to monitor the release of radiolabeled adenine, a technique shown to be a sensitive method to monitor drug-induced toxicity. The ACAT inhibitors were added for up to 48 h and at concentrations up to 2 micrograms/ml. These conditions resulted in an approximately 2-fold increase in adenine release. The increase in cell toxicity paralleled an increase in the cellular free cholesterol content. Reducing the cellular free cholesterol content, by the addition of extracellular acceptors, decreased the cytotoxic effects of the ACAT inhibitors. Addition of an intracellular cholesterol transport inhibitor, either progesterone or U18666A, together with CP-113,818 blocked the toxic effect of CP-113,818. These results suggest that ACAT inhibition of cholesterol-enriched macrophages increases cell toxicity due to the buildup of cellular free cholesterol. Removal of free cholesterol by the addition of extracellular cholesterol acceptors or by blocking intracellular sterol transport relieves the ACAT inhibitor-induced toxicity.