Immunosuppressive drugs common in clinical transplantation are known to have untoward effects on the vascular system. The effects of some drugs, notably cyclosporin A (CyA), have been studied on the vascular system, while those of others have not. In the vascular system, endothelial cells are the predominant cell type exposed to intravascular concentrations of immunosuppressive drugs. We therefore studied the effects of drugs common in clinical transplantation on endothelial cells in a capillary tube assay. The endothelial cells in the capillary tubes are morphologically more similar to those in the microvasculature than endothelial cells in monolayers. We studied the kinetics and extent of capillary tube formation and prostacyclin (PGI2) and endothelin-1 (ET-1) release from the in vitro capillaries to determine the morphological and biochemical effects of five immunosuppressive agents on endothelial function. We found a significant difference in the morphological and biochemical effects of the two common calcineurin inhibitors, CyA and tacrolimus (FK506) on capillary morphology in vitro. The former had a pronounced injurious effect on the morphology of the in vitro capillaries, while the latter did not. CyA also significantly increased ET-1 release by the capillaries, but FK506 did not. Mycophenolate mofetil (MMF) was the only other agent that had a moderately injurious effect on the morphology of the in vitro capillaries. Sirolimus (rapamycin) and dexamethasone, similar to FK506, had no effect on the capillary morphology. All these agents, except dexamethasone, increased PGI2 release. Our data suggest that CyA adversely affects the morphology of the microvasculature and that this is mediated, at least partly, by an increased ET-1 release by endothelial cells exposed to CyA. These findings describe a novel effect of CyA and MMF on endothelial cells that could be relevant to understanding the mechanisms of immunosuppressive drug-mediated endothelial injury in clinical transplantation.