Recent evidence suggests that an aldosterone synthase (AS) separate from the 11 beta-hydroxylase (11 beta-OHase) mediates the final step(s) in aldosterone synthesis in the rat. We have compared changes in AS and 11 beta-OHase mRNA levels with experimental maneuvers known to stimulate or suppress aldosterone secretion. In Exp 1, male rats were fed regular rat chow (group 1), a low sodium, high potassium diet (group 2), or a high sodium, low potassium diet (group 3). Northern analysis of adrenal capsular (zona glomerulosa) and decapsulated adrenal core (fasciculata-reticularis) tissues was performed with specific oligonucleotide probes for AS and 11 beta-OHase mRNAs, normalized with a cDNA probe for 18S ribosomal RNA (rRNA). There was a marked increase in capsular AS mRNA in group 2 rats compared to levels in group 1 (P < 0.0001) and group 3 (P < 0.0001) rats. Capsular As mRNA decreased (P < 0.05) in group 3 compared to that in group 1 rats. Adrenal core AS mRNA levels were quite low with all three diets. In contrast, capsular and core 11 beta-OHase mRNA levels did not change significantly with diet. In Exp 2, two groups served as controls (groups 1 and 3), and two groups received sc injections of repository ACTH (groups 2 and 4). Control and ACTH-treated rats were killed 3 h (groups 1 and 2) or 24 h (groups 3 and 4) after initial ACTH administration. Capsular and core AS and 11 beta-OHase mRNA levels were evaluated with and without normalization with 18S rRNA, cyclophilin, and glyceraldehyde phosphate dehydrogenase housekeeping probes. Capsular AS mRNA increased at 3 h (P < 0.05), but decreased at 24 h (P < 0.01). Nonnormalized adrenal capsular and core 11 beta-OHase mRNA levels were unchanged at 3 h, but increased significantly at 24 h (P < 0.05) in capsular tissue of ACTH-treated rats. However, in ACTH-treated rats, 18S rRNA and cyclophilin mRNA levels increased at 24 h in both capsular and core tissue (P < 0.01), and glyceraldehyde phosphate dehydrogenase mRNA levels increased in capsular tissue (P < 0.00001), resulting in a lack of change in normalized 11 beta-OHase mRNA. Changes in cholesterol side-chain cleavage were similar to those in 11 beta-OHase mRNA. These data provide further evidence that a separate AS plays a significant role in modulating aldosterone secretion in rodents.