5-Aminolevulinate synthase mRNA levels from different tissues were quantitated by Northern blot hybridization analysis utilizing the rat liver 5-aminolevulinate synthase cDNA clone as probe. A 5-aminolevulinate synthase mRNA species of size 2.3 kb was seen in all the tissues examined. Densitometric scanning of the autoradiographs demonstrated that the adrenal gland contained the largest amount of 5-aminolevulinate synthase mRNA. Levels corresponding to approximately 50% of this amount were found in the small intestine, lung, heart, muscle and testes. In the liver and kidney the level was approximately 25% of that found in the adrenal gland. These results demonstrate the housekeeping role of this gene. Dexamethasone treatment for 1 day or 5 days dramatically induced 5-aminolevulinate synthase mRNA levels in the liver and small intestine, and to a lesser extent in lung, heart, kidney and muscle. Nuclear run-off experiments suggest that a post-transcriptional mechanism predominantly contributes to the dexamethasone-induced increase in 5-aminolevulinate synthase mRNA levels observed in the liver. Interestingly, in the steroidogenic tissues of the adrenal gland and testes, there was a substantial decrease in 5-aminolevulinate synthase mRNA levels after dexamethasone administration but the mechanism of this control remains to be investigated.