Abstract: Objective To study the in vitro cleavage activity and the reaction conditions of the hammerhead ribozymes for the second exons of pro alpha 1Ⅰand Ⅲcollagen genes, and observe the effect of the two ribozymes on collagen synthesis by the fibroblasts in scar tissue. Methods The fragments of the second exons of pro alpha 1Ⅰand Ⅲcollagen genes were cloned in-to the plasmids pT-Ⅰand pT-Ⅲand labeled with ³²P during transcription to obtain the target RNA. The transcription products of the plasmids pT-gⅠand pT-gⅢcontaining specific ribozymes were incubated with the label target RNAs, respectively, un-der various conditions, and the results observed by electrophoresis autoradiography. The constructed ribozymes were trans-duced into cultured fibroblasts via liposomes for investigation of their effects on mRNA synthesis of type Ⅰand Ⅲcollagen protein by image analysis. Results The two ribozymes (rgⅠand rg Ⅲ) could efficiently cleave their target RNAs at both 37 ℃and 42 ℃, in the presence of Mg²⁺ within a relatively wide concentration range form 10 to 20 mmol/L. When the temperature was lowered from 65 ℃to 37 ℃and maintained at 37 ℃, the cleavage activity of the ribozymes reached the maximum. The expression of mRNA of Ⅰand Ⅲcollagen decreased accompanied by reduced collagen synthesis. Conclusion Hammerhead ribozymes for the fragments of the second exons of pro alpha 1Ⅰand Ⅲcollagen genes can cleave its target RNAs in vitro and effectively inhibit the collagen synthesis by the scar-derived fibroblasts.