Collagen synthesis in human fibroblasts was increased by the presence of 0.1M excess NaCl in the culture medium. Type analysis of the collagen produced by the cells showed the increase in the ratio of type III collagen to the total collagen. Similar NaCl effects were observed in several cell lines derived from different tissues. Sodium ion but not chlorine ion seems to be essential to the modulation of collagen production by the cells, because addition of CH3COONa induced similar effects but addition of KCl instead of NaCl resulted in specific inhibition of the collagen production.