Adrenocorticotropic hormone (ACTH)-induced hypertension in the rat is characterized by nitric oxide deficiency. Tetrahydrobiopterin (BH4) is an essential cofactor for the enzyme nitric oxide synthase and glucocorticoids have been reported to reduce cytokine-induced BH4 production. Accordingly we hypothesized that ACTH-induced hypertension would be reversed by BH4 supplementation. Male Sprague-Dawley rats (n = 33) were treated with BH4 in vehicle (10 mg/kg/day i.p.) or vehicle alone (5 mg/kg/day i.p. of ascorbic acid in 4 mM HCl) for 10 days. ACTH (0.2 mg/kg s.c.) or saline daily injection was started 2 days after BH4 or vehicle treatment and continued for 8 days. Systolic blood pressure (SBP) was measured on alternate days using the tail cuff method. Treatment with HCl, ascorbic acid or BH4 alone had no effect on SBP. In saline treated rats, neither BH4 nor its vehicle modified SBP. In ACTH treated rats, SBP was increased in both BH4 (from 128 +/- 6 to 142 +/- 4 mmHg, T0 to T10, P < 0.0005, one way ANOVA) and vehicle groups (from 127 +/- 3 to 158 +/- 7 mmHg, T0 to T10, P < 0.001, one way ANOVA). There was no significant difference in SBP between BH4 + ACTH treated and vehicle + ACTH treated rats. Thus, daily injection of BH4 (10 mg/kg i.p.) failed to prevent the development of ACTH-induced hypertension in rat.