Background— In the monocrotaline (MCT)-treated rat, there is marked stimulation of DNA synthesis and megalocytosis of pulmonary arterial endothelial cells (PAECs) within 3 to 4 days, followed by pulmonary hypertension (PH) 10 to 14 days later. Growing evidence implicates caveolin-1 (cav-1) in plasma membrane rafts as a negative regulator of promitogenic signaling. We have investigated the integrity and function of endothelial cell–selective cav-1α/raft signaling in MCT-induced PH. Methods and Results— Although PH and right ventricular hypertrophy developed by 2 weeks after MCT, a reduction in cav-1α levels in the lung was apparent within 48 hours, declining to ≈30% by 2 weeks, accompanied by an increase in activation of the promitogenic transcription factor STAT3 (PY-STAT3). Immunofluorescence studies showed a selective loss of cav-1α and platelet endothelial cell adhesion molecule-1 in the PAEC layer within 48 hours after MCT but an increase in PY-STAT3. PAECs with cav-1α loss displayed high PY-STAT3 and nuclear immunostaining for proliferating cell nuclear antigen (PCNA). Biochemical studies showed a loss of cav-1α from the detergent-resistant lipid raft fraction concomitant with hyperactivation of STAT3. Moreover, cultured PAECs treated with MCT-pyrrole for 48 hours developed megalocytosis associated with hypo-oligomerization and reduction of cav-1α, hyperactivation of STAT3 and ERK1/2 signaling, and stimulation of DNA synthesis. Conclusions— MCT-induced disruption of cav-1α chaperone and scaffolding function in PAECs likely accounts for diverse alterations in endothelial cell signaling in this model of PH.