We compared changes in the morphology of mitochondrial cristae with those in the blood and adrenal content of steroid hormones after the stimulation or inhibition of steroidogenesis. Rats were treated with adrenocorticotrophic hormone or angiotensin II to elicit steroidogenesis and with dexamethasone to inhibit it. Blood and adrenal glands were collected after several time intervals for measurements of steroids and their main intermediates. In the zona fasciculata, mitochondrial ultrastructure was investigated by high resolution scanning electron microscopy. We found that the morphometric data correlated well with the measurements of hyper- or hypo-activity of steroidogenesis over short periods of time (4 h) but not over longer observation times. A peculiar finding was that, contrary to previous reports, 11-deoxycortisol was present in adult rat adrenal tissue.