Metallothioneins, a group of cysteine-rich heavymetal binding proteins, are induced in the regenerating rat liver in response to the stimuli evoked by partial hepatectomy. We have investigated the expression of metallothionein genes and proto-oncogenes (c-fos, c-jun and c-myc), as well as specific localization of metallothionein in the liver cells after partial hepatectomy. Metallothionein mRNA was detected as early as 3 hr and reached a maximal level by 6 hr. Expression of the proto-oncogenes apparently preceded the elevation of metallothionein protein because the latter was maximal 18 hr after partial hepatectomy, followed by a decrease until 70 hr. Hepatocytes of the intact rat liver have metallothionein in the cytoplasm only. Interestingly, metallothionein was localized predominantly in the nucleus as early as 6 hr after partial hepatectomy, and the staining intensity of metallothionein became maximal at 15 hr, followed by detection in both the cytoplasm and nucleus at 24 hr or longer. The use of a confocal laser scanning microscope with both tissue sections and isolated nuclei has clearly shown that metallothionein immunofluorescence exists inside hepatocyte nuclei after partial hepatectomy. Expression of the proto-oncogenes c-fos and c-jun is elevated after partial hepatectomy, and the resultant heterodimer of gene products may contribute to the observed metallothionein gene induction. However, the observation that metallothionein protein levels were elevated until 18 hr after partial hepatectomy suggests that an alternative pathway for the induction of metallothionein gene expression may also be present. It is proposed that the specific localization of metallothionein in the nucleus may be involved in the early events associated with the compensatory cell proliferation that follows partial hepatectomy. (HEPATOLOGY 1993;18:1193-1201).