The compound, 2-methyl-1,2-bis-(3-pyridyl)-1-propanone (SU 4885) is regarded as a relative specific inhibitor of hydroxylation at C 11β of the steroid nucleus (Liddle, Island, Lance & Harris, 1958). Recently, however, evidence has accumulated suggesting an inhibitory action of SU 4885 on oestrogen biosynthesis (Kahnt & Neher, 1962; Griffiths, 1963; Giles & Griffiths, 1964; Földes, Koref, Fehér & Steczek, 1964). In the present study an attempt was made to obtain evidence for an effect of SU 4885 on oestrogen synthesis in vivo. Six female rats of the Wistar strain with a regular oestrous cycle, weighing 130–170 g., were injected s.c. with 10 mg. SU 4885 as the ditartrate salt, twice daily for nine consecutive days. They all showed continuous dioestrus from the 5th to 9th day after injection, when they were killed. In another experiment four female rats of the same strain, weighing 130–150 g., were given SU 4885 orally at a