Abstract Rats with transplants of Furth's mammotropic pituitary tumor (MtT-F4) have high plasma levels of ACTH, growth hormone and prolactin. Adrenals from normal rats and rats bearing the tumor for 2, 3 and 5 weeks were incubated with [ 14C]-pregnenolone in a time-course study. MtT-F4 rats showed an inhibition in the conversion to aldosterone and 18-hydroxy corticosteronc, with an increase in steroids produced mainly in the zona fasciculata-reticularis: corticosterone, deoxyeorticostcrone and 18-hydroxy deoxycorticosterone. Measurement of steroids in adrenal venous blood showed decreased aldosterone secretion ( P < 0.001 vs. controls at the 3rd and 5th weeks of the tumor growth) and an increase in corticosterone and deoxyconicosterone secretion at the 5th week ( P < 0.001 and P < 0.05 vs controls, respectively). A proportional higher increment in deoxycorticosterone secretion at the 5th week was found. Determination of mitochondrial 11β-hydroxylation of deoxycorticosterone showed higher basal activity in MtT-F4 adrenals than in controls ( P < 0.001); however, Krebs cycle intermediates at 10mM stimulated hydroxylation better in normal than in MtT-F4 mitochondria. The reaction was stimulated better in MtT-F4 than in controls by β-hydroxybutyrate and α-glycerophosphate. Thus, rats bearing the MtT-F4 showed (1) functional inhibition of the zona glomerulosa with concomitant hypcrfunction of the fasciculata-reticularis, and (2) abnormal regulation of the mechanisms supporting 11β-hydroxylation. These changes may be due to the high levels of ACTH circulating in these animals.