We have measured the location of embryonic and adult hemopoietic foci in the liver tissue of postnatal and adult phenylhydrazine-treated mice. Differentiation of acinar domains in liver tissue was made possible by carrying out succinate dehydrogenase histochemical reactions on liver cryostat sections. To determine the position of hemopoietic foci within the lobular gradient of the hepatocyte succinate dehydrogenase activity, this enzyme was measured in hepatocytes surrounding both portal and central veins and hemopoietic foci. Then, assuming the periportal succinate dehydrogenase activity value to be 1.00 ± 0.2, succinate dehydrogenase activity around postnatal hemopoietic foci was 0.65 ± 0.19, around phenylhydrazine-induced hemopoietic foci 0.83 ± 0.24 and around central veins 0.44 ± 0.11. Scaling the portal to central vein distance and taking 1 as the portal vein point and 0 as the central vein point, the relative position of hemopoietic foci, indirectly calculated from succinate dehydrogenase activity values, was 0.35 ± 0.13 in postnatal livers and 0.73 ± 0.12 in phenylhydrazine-treated adult livers. Hemopoietic foci frequencies varied according to both the origin and the liver acinar domain: in postnatal liver acini, it was 37.1% in zone 1, 22.8% in zone 2 and 40% in zone 3; in phenylhydrazine-treated adult acini, it was 89.4% in zone 1 and 10.6% in zone 2. Postnatal hemopoietic foci mainly occurred extrasinu-soidally, between hepatocytes and reticular-like cells, whereas adult hemopoietic foci were mostly intrasinu-soidal and closely associated to macrophage-like cells. Adult hemopoietic colonies specifically developed in the periportal microenvironment of liver acini, which contrasts with the random distribution of residual hemo-poiesis in postnatal mice. The biological conditions underlying phenylhydrazine-induced adult hemopoiesis are in the periportal regions of liver tissue.