Multienzyme branched-chain alpha-ketoacid dehydrogenase complex (BCKDH) catalyzes the regulatory step of oxidative catabolism of indispensable branched-chain amino acids (BCAA). The activity of the BCKDH complex is regulated by a reversible phosphorylation, end-product inhibition and by changes in the gene expression of BCKDH component enzymes. It has been shown previously that a high dose of bezafibrate (an agent added to rat chow at final concentration of 0.5%) changes mRNA levels of BCKDH-related enzymes and increases dephosphorylation of the complex leading to stimulation of liver BCKDH activity and the enhanced BCAA catabolism. The aim of the present study was to determine an in vivo effect of low, clinically relevant doses of bezafibrate on BCKDH activity in rat liver. Bezafibrate was administrated for 14 days by gastric gavage to Wistar male rats (fed low-protein chow; 8% protein) at one of the following daily doses of 5, 10 and 20mg/kgb.wt. The control group was given the vehicle (0.3% methylcellulose) only. The actual BCKDH and total BCKDH activities were assayed spectrophotometrically before and after incubation with a broad-specificity phosphatase, respectively. The mRNA levels of the selected genes (BCKDH catalytic subunits and regulatory enzymes) were quantified by means of semi-quantitative RT-PCR. Current catalytic activity of BCKDH (described as BCKDH activity state - the proportion of the BCKDH complex in its active dephosphorylated form) increased by 2.1 ± 0.2, 2.3 ± 0.2 and 2.7 ± 0.2 fold (p<0.01). Changes in BCKDH activity did not correspond with changes in mRNA levels of the complex catalytic subunits. Moreover, mRNA levels of regulatory enzymes remained unaltered. Initially bezafibrate caused a transient insignificant reduction in body weight, but it had no effect on the final body weight. The highest dose of bezafibrate induced hepatomegaly. In conclusion, these data indicate that under conditions of dietary protein restriction low, clinically relevant doses of bezafibrate have a similar adverse effect on rat liver BCKDH activity and BCAA degradation rate as the high experimental dose. Up-regulation of liver BCKDH activity by low doses of bezafibrate appears to result mainly from changes in phosphorylation status of the complex (increased dephosphorylation) and is not associated with elevations in mRNA levels of BCKDH enzymatic components.