The influence of the prototype aromatase inhibitor Aminoglutethimide (AG) and its analogue Rogletimide (RG) on peripheral aromatisation were investigated in 13 postmenopausal women with advanced breast cancer. Seven patients received AG 1,000 mg daily plus Hydrocortisone (HC) cover and six received RG as dose escalation of 200 mg bd, 400 mg bd and 800 mg bd. In vivo aromatase inhibition was investigated using the double bolus injection technique with [4-14C] oestrone ([4-14C]E1) and [6,7-3H] androstenedione ([6,7-3H]4A) followed by a 96 h urine collection. The labelled urinary oestrogens were separated and purified by chromatography and HPLC. Plasma oestradiol (E2) was also measured. AG mean aromatase inhibition was 90.6% +/- 1.8 s.e.m. and E2 suppression 75.7% +/- 7.3 s.e.m. RG mean aromatase inhibition was 50.6% +/- 9.8 s.e.m. at 200 mg bd, 63.5% +/- 5.7 s.e.m. at 400 mg bd and 73.8% +/- 5.8 s.e.m. at 800 mg bd. E2 suppression was 30.7% +/- 9.5 s.e.m., 40.2% +/- 10.3 s.e.m. and 57.6% +/- 9.2 s.e.m. respectively. These results confirm the efficacy of AG as an aromatase inhibitor. RG produced dose dependent E2 suppression and aromatase inhibition, but even at the maximum tolerated dose of 800 mg bd had sub-optimal aromatase inhibition and oestradiol suppression compared with AG.