The molecular characteristics of purified pepsin solubilized collagen from rat liver was studied in control and dimethylnitrosamine administered animals. The alpha- and beta-chains of purified pepsin solubilized liver collagen were separated by subjecting the denatured collagen to SDS-polyacrylamide gel electrophoresis. The alpha 1(III) chains were resolved from the alpha 1(I) chains by interrupted electrophoresis with delayed reduction of the disulfide bonds of type III collagen. The aldehyde content of the purified pepsin solubilized collagen was estimated in control and experimental samples in order to assess the extent of collagen cross-links. Fibril formation curves were studied with purified pepsin solubilized collagen to see the rate of formation of cross-links within the fibrillar mesh. The results of the unreduced electrophoretic studies revealed a significant increase in the beta-subunit of type I collagen with a remarkable decrease of alpha/beta ratio in DMN treated animals. Reduction with beta-mercaptoethanol indicated the presence of type III collagen in the electrophoretic field with a proportionate increase on the 21st day. A significant increase in the aldehyde content and an increased rate of fibril formation were noticed in DMN induced fibrotic liver collagen. The data of the present investigation revealed that the DMN induced fibrotic liver collagen is more cross-linked than normal liver collagen and the deposition of type III collagen in more prominent than type I collagen in early fibrosis.