OBJECTIVE: To study the effect of catechin on transforming growth factor β1 (TGF β1) expression in rats with the nephrotic syndrome. METHODS:Twenty female SD rats were randomly assigned into the normal control group, un treated nephrotic group, dexamethasone treated nephrotic group and catechin treated nephrotic group. The concentrations of serum albumin (Alb), total protein (TP), trilyceride (TG), blood urea nitrogen (BUN) and TGF β1, and the excretion of 24 h urinary protein were detected by the biochemical assay; the TGF β1 and TGF β1 mRNA expressions in renal intrinsic cells were measured by the immunohistochemical technique and in situ hybridization (ISH) respectively; a semiquantitative score was used to evaluate the degree of glomerular and tubulointerstitium. RESULTS: The serum total TGF β1 [( 59.40 ± 8.12 ) μg/L] and activated TGF β1 [( 47.56 ± 9.88 ) μg/L], TGF β1 and TGF β1 mRNA expressions in renal intrinsic cells [( 45.1 ± 2.0 ) % and ( 51.6 ± 3.2 ) %] in the catechin treated nephrotic group were lower than those in the un treated nephrotic group [( 127.78 ± 16.11 ) μg/L, ( 93.79 ± 12.45 ) μg/L, ( 56.9 ± 3.5 )% and ( 60.4 ± 4.8 )%, respectively](all P< 0.01 ). The serum Alb concentration increased significantly and the TG content obviously decreased in the catechin treated nephrotic group compared with the un treated nephrotic group [( 11.28 ± 4.18 ) g/L vs ( 1.46 ± 0.71 ) g/L; ( 2.89 ± 0.64 ) mmol/L vs ( 6.02 ± 0.90 ) mmolo/L] (both P< 0.01 ). The pathologic lesions of kidneys were remissive in the catechin treated nephrotic group compared with the nephrotic group. CONCLUSIONS: Catechin may alleviate kidney lesions and slow down the progression of kidney lesions by inhabiting the expression of TGF β1 mRNA and decreasing TGF β1 expression in renal intrinsic cells and serum activated TGF β1 expression effectively.