1. A series of experiments was carried out on 3 separate groups of male Long Evans rats, chronically instrumented for the measurement of regional haemodynamics, to compare the effects of NG,NG, dimethyl-L-arginine (ADMA) and NG-monomethyl-L-arginine (L-NMMA), and their reversibility by the nitric oxide donors, S-nitroso-N-acetyl-penicillamine (SNAP), S-nitroso-glutathione (SNOG), sodium nitroprusside (SNP), and the vasodilator, hydralazine. 2. As previously reported for L-NMMA, ADMA (1-100 mg kg-1) caused dose-dependent pressor and bradycardic effects, accompanied by renal, mesenteric and hindquarters vasoconstrictions. The magnitude and duration of these effects were similar for ADMA and L-NMMA, consistent with their being equipotent inhibitors of nitric oxide synthase. 3. Infusion of SNAP or SNOG (300 micrograms kg-1 h-1) after injection of ADMA or L-NMMA (100 mg kg-1) reversed the pressor but did not abolish the vasoconstrictor, effects of ADMA or L-NMMA. However, a higher dose of SNAP (3 mg kg-1 h-1) caused complete reversal of the pressor and mesenteric haemodynamic effects of ADMA (100 mg kg-1), although its renal and hindquarters vasoconstrictor effects were not abolished. 4. Infusion of SNP (300 micrograms kg-1 h-1) after administration of L-NMMA (100 mg kg-1), caused complete reversal of its pressor and mesenteric and hindquarters haemodynamic effects, and reduced substantially its renal vasoconstrictor action; hydralazine (7.5 mg kg-1 h-1) was almost as effective as SNP in reversing all these variables.(ABSTRACT TRUNCATED AT 250 WORDS)

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