To facilitate the quantitation of the steady-state levels of tyrosine hydroxylase (TH) mRNA, we have developed a specific, rapid, and sensitive solution hybridization assay. By use of this assay, the induction of TH mRNA by protein synthesis inhibitors (PSIs) in the rat adrenal was investigated. Treatment with PSIs, either cycloheximide (CHX) or anisomycin (ANI), increased the levels of TH mRNA in the adrenal. The levels of adrenal TH mRNA were increased 1 h after CHX and had increased by approximately threefold over saline control [from 3.85 +/- 0.4 (SE) to 11.5 +/- 2 (SE) pg/mug RNA] at 6 h after treatment. A similar time course was observed after ANI treatment. These in vivo kinetics cannot be accounted for solely by the stabilization of TH mRNA. Northern blot analysis indicated that the size of the TH mRNA in the adrenal was not altered by PSIs. The CHX induction of adrenal TH mRNA was dose-dependent in the range of 2 to 4 mg/kg, sc, and was not further increased by a dose of 50 mg/kg, sc. Unilateral adrenal denervation did not alter the CHX induction of TH mRNA in medullae. These results suggest that PSIs exert a direct effect on the adrenal medulla. These effects of PSIs may be mediated by a labile repressor which regulates adrenal TH gene expression in vivo.