The pyrrolizidine alkaloid, monocrotaline (MCT), is a plant toxin that causes injury to the vasculature of the lungs and pulmonary hypertension in animals. To produce lung injury, MCT is bioactivated in the liver by cytochrome P450 monooxygenases to pyrrolic metabolites which travel via the circulation to the lungs, where they cause injury by unknown mechanisms. One putative metabolite of MCT is monocrotaline pyrrole (dehydromonocrotaline, MCTP), a moderately reactive, bifunctional alkylating agent. A single, iv injection of chemically synthesized MCTP into rats causes delayed and progressive lung vascular injury and pulmonary hypertension similar to that caused by MCT itself. Since pulmonary vascular endothelium is likely an important target of MCTP in vivo, the effects of MCTP on cultured endothelium were studied. A single application of MCTP to confluent monolayers of cultured endothelium from bovine pulmonary artery results in release of lactate dehydrogenase, some cell detachment from the growth surface and markedly altered morphology of remaining viable cells. These effects are dose-dependent and, as in vivo, are delayed in onset (1-2 days) and progressive. In endothelial cells of porcine origin, these particular responses to MCTP are also apparent but much less pronounced. Inhibition of proliferation of cells plated at low density occurred in both cell types at nominal MCTP concentrations (0.5 micrograms/ml) that were not overtly cytotoxic. These results indicate that MCTP causes a direct, dose-dependent injury to pulmonary vascular endothelium in culture that is delayed and progressive and suggest a mechanism by which MCT may act in vivo to cause lung injury and pulmonary hypertension.