1. Rats were bilaterally adrenalectomized and treated for periods of up to 32 days with doses of dexamethasone equivalent to physiological and supraphysiological endogenous glucocorticoid secretion. Aortic production of 6-oxoprostaglandin F 1α (6-oxo-PGF 1α ) was measured by radioimmunoassay of the supernatant fluid from rings of aorta incubated in vitro. 2. Up to 32 days after adrenalectomy, neither adrenalectomy nor dexamethasone (2 μg/day and 20 μg/day) influenced aortic 6-oxo-PGF 1α production. 3. High doses of dexamethasone (60 μg/day for 6 days) in the presence or absence of mineralocorticoid (11-deoxycorticosterone, 600 μg/day) had no effect on aortic 6-oxo-PGF 1α production. 4. in contrast to the lack of effect of dexamethasone administered in vivo , incubation of aortic discs for 5 h with dexamethasone (1 μmol/l) in vitro caused inhibition of aortic 6-oxo-PGF 1α production to 23% of control levels. 5. Alteration in endogenous glucocorticoid secretion is unlikely to be involved in the physiological regulation of aortic 6-oxo-PGF 1α , production.