Increased medial arterial thickness is a structural change in pulmonary arterial hypertension (PAH). The role of smooth muscle hypertrophy in this process has not been well studied. Bone morphogenetic proteins (BMPs), transforming growth factor (TGF)-β1, serotonin (or 5-hydroxytryptamine; 5-HT), and endothelin (ET)-1 have been implicated in PAH pathogenesis. We examined the effect of these mediators on human pulmonary artery smooth muscle cell size, contractile protein expression, and contractile function, as well on the roles of glycogen synthase kinase (GSK)-3β and p70 ribosomal S6 kinase (p70S6K), two proteins involved in translational control, in this process. Unlike epidermal growth factor, BMP-4, TGF-β1, 5-HT, and ET-1 each increased smooth muscle cell size, contractile protein expression, fractional cell shortening, and GSK-3β phosphorylation. GSK-3β inhibition by lithium or SB-216763 increased cell size, protein synthesis, and contractile protein expression. Expression of a non-phosphorylatable GSK-3β mutant blocked BMP-4-, TGF-β1-, 5-HT-, and ET-1-induced cell size enlargement, suggesting that GSK-3β phosphorylation is required and sufficient for cellular hypertrophy. However, BMP-4, TGF-β1, 5-HT, and ET-1 stimulation was accompanied by an increase in serum response factor transcriptional activation but not eIF2 phosphorylation, suggesting that GSK-3β-mediated hypertrophy occurs via transcriptional, not translational, control. Finally, BMP-4, TGF-β1, 5-HT, and ET-1 treatment induced phosphorylation of p70S6K and ribosomal protein S6, and siRNAs against p70S6K and S6 blocked the hypertrophic response. We conclude that mediators implicated in the pathogenesis of PAH induce pulmonary arterial smooth muscle hypertrophy. Identification of the signaling pathways regulating vascular smooth muscle hypertrophy may define new therapeutic targets for PAH.