In the monocrotaline (MCT) rat model of pulmonary hypertension, the pulmonary vascular endothelium is thought to be the early target of the bifunctionally reactive metabolite monocrotaline pyrrole (MCTP). In previous studies, bovine pulmonary arterial endothelial cells (BPAEC) exposed to MCTP exhibited inhibition of proliferation. Since other compounds that crosslink DNA lead to cell cycle alterations, we utilized BPAEC to correlate the effects of MCTP on the cell cycle with the extent of covalent binding of [14C]MCTP to BPAEC DNA. Dose response (0.0 to 50.0 micrograms MCTP/ ml) and 96-hr time course (5 micrograms MCTP/ml low dose or 34.5 micrograms MCTP/ml high dose) studies were carried out followed by flow cytometric cell cycle analysis. High concentrations of MCTP caused cell cycle arrest in S phase, beginning by 24 hr, while an S phase delay was observed at low concentrations, but progressed to a G2 + M phase arrest by 48 hr. Covalent DNA binding (34.5 micrograms/ml of [14C]MCTP incubated with BPAEC) occurred within 1 hr and progressively increased through 96 hr. In conclusion, covalent binding of MCTP to DNA is associated with cell cycle arrest; however, the position of cell cycle arrest is dependent on dose, with an S phase arrest at high concentrations and a G2 + M phase arrest at low concentrations of MCTP. The mechanism by which MCTP induces proliferative inhibition could be cell cycle arrest.