EPO increased bioavailability of endothelial NO in brain microvascular endothelial cells. Bioavailability of BH4 was significantly increased in GTPCH-I siRNA treated BMECs exposed to EPO (A, * P<0.05, n=6). EPO also attenuated superoxide anion production in GTPCH- I siRNA treated BMECs (B, * P<0.05, ** P<0.01, n=8). Levels of total nitrite was significantly increased in GTPCH-I treated BMECs exposed to EPO (C, * P<0.05, ** P<0.01, n=6).