To explore the effect of dimethyl 4-(2-chlorophenyl)-1,4-dihydro-2,6-dimethyl-3,5-pyridinedicarboxylate (DCDDP) on pulmonary hypertension (PH) induced by monocrotaline (MCT), the parameters of pulmonary hemodynamics, the contents of endothelin-like immunoreactivity, nitric oxide (NO), malondialdehyde, and superoxide dismutase in plasma and pulmonary homogenate were measured. DCDDP was administered in 5, 50, and 500 µg · kg¹ · day¹ ip doses, once a day for 28 days. The antiserotonin effect of DCDDP was investigated by using immunohistochemistry, image analysis, and cell culture technique. The results showed that pulmonary arterial pressure was significantly dropped and pulmonary resistance was decreased in DCDDP groups, compared with the MCT group. DCDDP had no influence on endothelin-like immunoreactivity levels in plasma and pulmonary homogenate but reduced the contents of NO, superoxide dismutase, and malondialdehyde in pulmonary homogenate enhanced by MCT. DCDDP also significantly inhibited the increase in numbers of 5-hydroxytryptamine (5-HT) and 5-HT receptor-positive cells in pulmonary tissue of PH rats induced by MCT. The proliferation and contraction of pulmonary arterial smooth muscle cells and the increase in concentration of free Ca²⁺ in them evoked by 5-HT were inhibited significantly by DCDDP. The results suggest that DCDDP reduces the production of free radicals and content of 5-HT and 5-HT receptor and the increase in NO in pulmonary tissue, which underlies the mechanisms of DCDDP against MCT-induced PH.