In a previous paper, experiments were described in which the collagen content of the rat's liver, regenerating after partial hepatectomy, was used as an indication of the growth of supporting tissue (Harkness, 1952 a). Regeneration of collagen was found to lag far behind that of the liver as a whole, and was apparently incomplete 3 weeks after partial hepatectomy when the liver parenchyma is fully regrown. Collagen, however, was estimated chemically as a whole, and, consequently, the morphological significance of the changes was not clear. True collagen, judged by staining properties, is present in the peritoneal capsule of the liver, and in the walls and tissues surrounding the blood vessels and bile ducts. In addition, there is a network of reticulin in walls of the hepatic sinusoids. The available evidence indicates that this material is similar to collagen chemically (Bowes & Kenten, 1949), and the term collagen will be used to include it.We thought that the regeneration of collagen at 3 weeks, although it did not reach the original level, might nevertheless be complete in the sense that it had reached an equilibrium state representing a somewhat altered liver structure. There are features of 3-week regenerated liver which lead one to suspect that it might be constructed with less than the normal proportion of collagen. Thus the lobes become more rounded with less relative surface area, so one might expect the capsular collagen to increase less than the weight of the liver. The fact that the lobes are more rounded should also make it unnecessary for the main vascular tree to expand as much proportionately as the weight. On the other hand, one might expect the sinusoids to reach their normal size relative to the parenchymal cells and hence to increase approximately in proportion to the weight of the liver. In order to clarify our previous results we have therefore separated the collagen in the liver into three fractions-capsular, vascular and parenchymal, the latter representing the collagen of the vessels most closely associated with the parenchyma-and estimated the collagen in these separate fractions at 3 and 6 weeks after partial hepatectomy. At 6 weeks the