AIM: To investigate the effect of L-arginine(L-arg) on the production of collagens of human mesangial cells. METHODS: Radioimmunoassay, hydroxyproline colorimetric assay and reverse transcription polymerase chain reaction (RT-PCR) were used to determine procollagen Ⅲ, total collagen level in the supernatant and expression of collagen Ⅳ mRNA in human mesangial cells. RESULTS: L-arg significantly inhibited the production of procollagenⅢ, total collagen in the supernatants ( P