Exposure of a trout gonadal fibroblast (RTG-2) cell line to ZnCl 2, CdCl 2 and CuCl 2 resulted in differential levels of accumulation of metallothionein (MT) mRNA. ZnCl 2 being the most effective agent induced MT mRNA in 3 h, with 172-fold induction after 48 h and continued accumulation up to 144 h. Following CdCl 2 treatment, mRNA could be detected after 24 h, reaching peak levels at 72 h. Furthermore, trout MT mRNA could be detected up to 8 days after withdrawal of extraneous ZnCl 2. Using a novel technique of primer extension and DNA sequencing with total RNA as template, specificity of the trout MTa and MTb gene-specific primers was established. Primer extension studies revealed a higher response of MTa to ZnCl 2 and CdCl 2 compared to MTb. Insensitivity of MT mRNA induction to cycloheximide suggested that induction by the metals was independent of de novo protein synthesis. However, simultaneous exposure of cells to actinomycin D and metals completely inhibited MT mRNA synthesis implying control at the transcriptional level.