The metabolic activity of lysosomes was monitored by assaying the lysosomal enzymes. DNase, RNase, acid phosphatase and cathepsin in rat liver regenerating after partial hepatectomy at various intervals after the operation. Results of the present study demonstrate that the activities of lysosomal enzymes are increased significantly during the process of rapid regeneration, especially at 24 h and 48 h. A small rise in enzymatic activities was noticed at 36 and 72 h of regeneration, leading towards normalisation at 168 h. Administration of tin(II) tartrate (25 mu mol Sn2+/100 gm body wt.) significantly increased the DNAse and RNAse activity at 24 h of the regeneration, while a decrease was noticed in acid phosphatase activity. An increase was also found in the catheptic activity of tin-treated hepatectomized rats. It can thus be concluded that an increased metabolic activity of lysosomes is reflected by the increased activity of its hydrolytic enzymes during the regeneration, and tin at low concentrations is helpful to this process. Further work is in progress to gain insight into the other metabolic activity of the lysosomes during the process of regeneration.