Oestrogens in high doses have been shown to be potent reticuloendothelial stimulants (Nicol, Bilbey, Charles, Cordingley & Vernon-Roberts, 1964). In the course of an investigation into the cellular mechanism of this effect, it seemed relevant to study the distribution throughout the reticuloendothelial system (RES) of tritiated oestrogens administered parenterally. By chance, localization to peripheral autonomic nervous tissue was observed. Since it is known that oestrogens are selectively taken up by certain areas in the CNS, notably the hypothalamus, wherein resides higher autonomic control, and since in these sites oestrogens have a significant effect on behaviour (see Harris, 1965; Michael, 1965; Palka & Sawyer, 1966) it seemed that this chance morphological observation might have functional significance and should therefore be reported. The animals used were white mice (20–25 g.) from a closed colony kept in the University of Sheffield animal house. [6,7-3H]Oestradiol (Radiochemical Centre, Amersham) was administered s.c. or