The relationship of thionein synthesis rates to translatable cytoplasmic thionein mRNA levels was investigated for the first time in a cultured cell system. Thionein synthesis was induced in Cdr, a cadmium-resistant variant of CHO, by exposure to 2 microM CdCl2. Following a short (1.5 hr) lag, thionein synthesis increases to a rate that is at least 30 times the uninduced rate 7-8 hr after addition of Cd++. This increase is blocked by the coincident addition of a actinomycin D. Cytoplasmic thionein mRNA levels, measured by translation in a modified wheat germ system, increase rapidly following induction to values approximately 25 times uninduced levels within 6-8 hr. The increase in thionein mRNA precede proportionate increases in thionein synthesis by 0.5-1.0 hr. Continued exposure to Cd++ results in a decreased thionein synthesis rate after 8 hr. By 30 hr, the rate is one-half that seen 6-8 hr after induction. Removal of Cd++ after 8 hr results in a rapid decrease in thionein synthesis (t 1/2 approximately 4 hr). Both decreases are inhibited by the addition of actinomycin. In all instances--induction, deinduction, and actinomycin-mediated "super-induction"--translatable thionein mRNA levels and thionein synthesis rates increase, decrease, or are maintained coordinately. The results suggest that thionein synthesis in Cdr is controlled primarily by the level of translatable cytoplasmic thionein mRNA.