The gut-blood barrier (GBB) controls the passage of nutrients and bacterial metabolites from the intestinal lumen to the bloodstream. Several studies suggest that trimethylamine N-oxide (TMAO), a liver metabolite of gut bacteria-produced trimethylamine (TMA), is a marker of cardiovascular risk. However, the mechanisms of the increase are not clear. The aim of the study was to evaluate the effect of heart failure (HF) on plasma level of TMA and TMAO in rats. The study was performed on 58-week-old Spontaneously Hypertensive Heart Failure rats (SHHF, n=9) and Wistar-Kyoto rats (WKY, n=7). Firstly, water-electrolyte and energy balance was evaluated. Subsequently, rats were anesthetized with urethane (1.5 g/kg b.w. i.p.) and underwent echocardiography. Next, the left femoral artery was cannulated for arterial blood pressure measurement, and standard needle electrodes for ECG recordings were connected (Biopac MP160 system). For a direct evaluation of left ventricle pressure, Millar Mikro-Tip SPR-320 (2F) pressure catheter was used. After the hemodynamic recordings, blood from the right ventricle of the heart was taken for biochemical analysis and rats were killed by decapitation. Tissues and the colon content (stools) were harvested for histological and biochemical analysis, respectively. There was no significant difference in body weight and food intake between WKY and SHHF. There was a trend towards higher water intake in SHHF than in WKY. WKY showed no pathological changes in the circulatory system. In contrast, SHHF showed hypertrophic cardiomyopathy with histopathological picture of transition from hypertrophic to dilated cardiomyopathy, reduced systolic function and diastolic functions. Specifically, SHHF had a significantly increased heart mass (80% increase), increased diameters of intraventricular septum and posterior heart wall, decreased stroke volume and ejection fraction and increased + dP/dt and – dP/dt. Moreover, SHHF had increased plasma NT-proBNP, increased plasma sodium and urea, decreased sodium excretion, significantly increased plasma TNF-α and vasopressin levels. Finally, in comparison to WKY, SHHF showed a significantly higher blood plasma level (mean±SE, µM) of TMA (81.04±5.19 vs 152.97±20.26) and TMAO (4.57±0.39 vs 6.70±0.68). There was no significant difference in stool concentration of TMA between WKY (273.65±23.11) and SHHF (284.81±16.64). However, SHHF showed a significantly higher blood-to-stool ratio of TMA concentration (0.56±0.11 vs 0.30±0.03) suggesting a greater gut-to-blood penetration of TMA. This was accompanied by a significantly decreased intestinal blood flow and significantly smaller thickness of the colonic mucosa in SHHF rats. In conclusion, rats with HF show increased plasma level of TMA and TMAO, which may results from increased gut-to-blood penetration of TMA. Therefore, we would speculate that increased plasma TMAO level in cardioavsulcar diseses is due in part to alterations in the GBB.