Heat production by brown adipose tissue (BAT) is important for thermoregulation in a cold environment. During thermogenesis, oxygen utilization increases, with an associated rise in free radical generation. Our objective was to investigate the expression of metallothionein (MT), which is thought to have an antioxidant role in BAT of rates transferred from 25 to 6 degrees C for 6 or 24 h or maintained at 25 degrees C throughout the study (control group). For comparison, MT expression was also measured in white adipose tissue (WAT), liver, and kidney. MT-1 mRNA and 18S rRNA were measured by Northern blotting using specific digoxigenin-labeled antisense oligonucleotide probes with chemiluminescence detection, and MT-1 protein was determined by radioimmunoassay. MT-1 mRNA in BAT increased after 6 h, and the mRNA level after 24 h was equivalent to that in liver 6 h after injection of rats with 10 mg Zn/kg. By 24 h, liver and kidney MT-1 protein had increased relative to the controls by 3- and 1.4-fold, respectively, but in BAT the relative induction was 16-fold. Zn injection did not affect BAT MT-1. As with MT-1 protein, Zn in BAT increased only after 24-h cold exposure. WAT MT-1 was not affected by any treatment. It is concluded that could exposure induces MT-1 in BAT, but in contrast to other tissues induction may be independent of Zn.