Effects of 5-hydroxytraptamine (5-HT) on growth of normal and irradiated NIH/3T3 fibroblasts were observed by using MTT colorimetry,a silver colloid technique of AgNORs, and immunohistochemical methods were used to estimate type III collagen and DNA synthesis. The results showed that,when 5-HT of 2×10,-6,mol/L was used,the counts of normal and irradiated fibroblasts increased by 1.5 times as many as that of control group; proliferative activity (expressed by OD values) of the fibroblasts were about 1. 3 and 1.6 times that of control value; the number of AgNORs granules elevated to 2.5 times that of control value. In addition,5-HT of 1.0×10,-6,mol/L also induced the increments of percentage of S-phase fibroblasts and type III collagen positive fibroblasts,both of them were 2.7 and 2.1 times that of control group,respectively.