Previous in vivo and in vitro studies have reported a variety of glucocorticoid effects on the synthesis and secretion of immunoreactive atrial natriuretic peptide (ir-ANP) into plasma. To further define glucocorticoid modulation of ir-ANP, we have measured ir-ANP levels in plasma and the four cardiac chambers, and tissue ANP mRNA levels, in intact rats and adrenalectomized rats with or without dexamethasone treatment for 1, 2, 4, 8 and 16 days. Plasma levels fell by 50% between 8 and 16 days post-adrenalectomy; in contrast, dexamethasone treatment caused a 3-fold rise in plasma ANP 1-2 days post-adrenalectomy, with levels gradually returning to control by day 16. Circulating forms of ANP were unchanged by adrenalectomy or dexamethasone treatment, as were atrial ANP concentrations and ANP mRNA levels. Left ventricular ANP concentrations rose with dexamethasone treatment, and ventricular ANP mRNA levels changed in parallel with those of circulating ANP. The in vivo effect of glucocorticoids (at moderate rather than very high doses) on ANP synthesis and secretion thus appears to be predominantly but not exclusively upon the left ventricle rather than the atria.