This study investigated whether the riboflavin analogs, 7,8-dimethyl-10-formylmethyl isoalloxazine (FMI) and 7,8-dimethyl-10-(2'-hydroxyethyl) isoalloxazine (HEI), are effective antihypertensive agents in mineralocorticoid-induced or deoxycorticosterone acetate (DOCA)-salt hypertension. These studies are based on our previous observation tht aldosterone enhances the biosynthesis of renal flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) from riboflavin, and that FMI and HEI competitively inhibit conversion of riboflavin to FMN and reabsorption of Na+ in the kidney of adrenalectomized rats. When 1.6 mg of FMI or HEI were administered simultaneously with 3.0 mg of DOCA, the tail systolic blood pressure (SBP) of unanesthetized rats rose only to 136 +/- 5 mm Hg (standard error of the mean, SEM) compared to 163 +/- 5 mm Hg during DOCA therapy alone (p less than 0.0005). This hypotensive effect of FMI or HEI was noted after the fourth week of treatment and persisted through the ninth week. The rats tolerated the medication well and had no signs of riboflavin deficiency. DOCA administration alone resulted in a 24% increase in iliopsoas muscle Na+ concentration (p less than 0.0005), and a 0.8% increase in the water content of the muscle (p less than 0.025), suggesting a positive Na+ balance. Administration of FMI or HEI blunted the ability of DOCA to increase muscle Na+ concentration (p less than 0.025), water content (p less than 0.01). HEI treatment of the Kyoto strain of spontaneously hypertensive rats (SHR) did not lower their mean SBP. Thus it appears that the hypotensive actions of FMI or HEI are closely associated with their ability to modify the effects of mineralocorticoids on NA+ balance.