Humans risk inadvertent intraperitoneal or intravenous exposure to formaldehyde (HCHO), commonly used for disinfection of implanted or extracorporeal medical devices. Various chemical and physical stresses are known to induce hepatic metallothionein. This study examined the effect of acute parenteral administration of HCHO on induction of hepatic metallothionein synthesis. Adult male CF1 mice were administered HCHO ip and hepatic metallothionein was quantified by the cadmium-radioassay method. HCHO (50 mg/kg) increased hepatic metallothionein as early as 8 hr after dosing with maximal levels (27-fold increase) occurring at 72 hr. Metallothionein concentrations were elevated (15-fold) 24 hr after 50 or 100 mg HCHO/kg but not at lower dosages. Concomitant elevations in hepatic zinc and copper content were observed. No increases in metallothionein were observed in kidney, pancreas, or intestine 24 hr after HCHO administration (100 mg/kg, ip). Induction of metallothionein by HCHO may reflect direct de novo synthesis since the response was abolished by pretreatment with the RNA synthesis inhibitor, actinomycin D. HCHO induction of metallothionein also does not appear to be mediated by stress-induced release of corticosteroids or catecholamines from the adrenal since the response was unaltered in adrenalectomized mice. Interference with the glutathione (GSH)-dependent oxidation of HCHO by reducing hepatic GSH concentrations to 40% of control after a 2-hr pretreatment with phorone decreased the metallothionein induction response to HCHO by 33%. This result suggests that the induction may be partially due to a HCHO metabolite, e.g., formate. Confirmation of metallothionein synthesis was obtained following spectral and chromatographic analysis. Thus, HCHO and/or a metabolite produces a marked increase in hepatic metallothionein and alters hepatic zinc and copper homeostasis, all of which are transient responses. Although HCHO was only mildly hepatotoxic at the highest dose (as evidenced by an increase in plasma alanine aminotransferase activity), such changes in metallothionein synthesis and essential metal homeostasis may be part of a cellular repair mechanism operant after acute toxic cell injury.