Background and purpose: Elevated plasma total homocysteine (tHcy) level has been established as an independent risk factor for cardiovascular diseases. Danshensu, an active ingredient of Salvia miltiorrhiza, shows wide cardiovascular benefit. However, In terms of Its own methylation, danshensu could elevate tHcy level, which would act against its cardiovascular benefit, thus posing a 'therapeutic paradox'. As this paradox has not been fully assessed, we have evaluated the effects of danshensu on tHcy levels to uncover the underlying mechanisms. Experiment approach: We evaluated the influence of danshensu on homocysteine (Hey) metabolism In rats with normal tHcy levels and in rat models of elevated tHcy (single intravenous methionine loading model and a hyperhomocystelnemic model after 3 weeks methionine dosing, with and without 3 weeks of danshensu treatment). We also quantified some metabolic intermediates (S-adenosyl methionine, S-adenosyl-l-homocysteine, cysteine and glutathione) relevant to Hey metabolism In rat liver and kidney. Key results: Acute treatment with a single dose of danshensu In rats with normal tHcy did not change plasma tHcy. In contrast, danshensu significantly lowered tHcy in rats with elevated tHcy. The relatively higher cysteine and glutathione levels after treatment with danshensu indicated that Its tHcy-lowerlng effect was via increased activity of the trans-sulphuration pathway. Conclusions and Implications: Our results suggested that danshensu may act both acutely to increase trans-sulphuration and after chronic exposure to up-regulate the activity of the trans-sulphuration enzymes. The tHcy-lowering effect of danshensu is another cardiovascular benefit provided by S. miltiorrhiza and suggests a potential tHcy-lowering therapy. © 2009 The British Pharmacological Society All right reserved.