ABSTRACT— By means of staining with Sirius Red F3BA in a saturated picric acid solution, the collagen contents of rat livers with varying degrees of fibrosis have been measured quantitatively in fixed and sectioned material, using both histophotometry in situ and extraction of bound dye with colorimetric analysis. These findings have been correlated with chemical assays of the hydroxyproline content in homogenates from the same livers. It appears that a highly significant correlation exists between both section-based analysis methods and the hydroxyproline content, the Spearman-Rank correlation coefficients being virtually identical. For analysis of collagen accumulation in rat liver, both section-based methods seem to be useful and reliable, the extraction method giving the quickest results for large-scale screening, and the histophotometric method being more appropriate to take readings in selected areas. With human liver material, indications have been obtained for the existence of large sampling errors due to inhomogeneous distribution of collagen deposits. Using the extraction method, no significant changes could be observed in the volume density of collagen during postnatal growth from 1 week to 21 months in rat liver: only on the third day after birth was a higher value of collagen/total protein obtained, possibly due to a higher water content of the hepatocytes. Partial hepatectomy was found to have no influence at all on the collagen content of rat liver during the period of restorative growth or after it.