SUMMARY Unit activity from the hypothalamus and EEG patterns from the cerebral cortex of rats rendered persistently oestrous by exposure to continuous illumination were analysed during the acute infusion of various progesterone preparations. Infusion of 400 μg. of progesterone in either 0·1 ml. propylene glycol or ethanol caused dramatic changes in the spike activity of many hypothalamic units and precipitated a reproducible change in the EEG pattern. The infusion invariably caused a prolonged synchronization of the EEG, lasting 30–40 min. (i.e. an EEG pattern of large-amplitude slow waves). In addition, a period of high-amplitude high-frequency EEG waves occurred 2–4 min. after the infusion. These EEG changes were usually concurrent with any changes which occurred in the spike activity of hypothalamic units. Similar changes were observed after the infusion of the control materials, i.e. 0·1 ml. of propylene glycol or ethanol. The inclusion of progesterone did not produce changes in brain activity which were substantially different from the control infusions. Infusion of up to 400 μg. of progesterone in propylene glycol or ethanol did not change the specific response of hypothalamic units to probing the vaginal parts of the cervix. On the other hand, this treatment frequently abolished the non-specific response (i.e. a response which is related to the period of EEG arousal) of these units for a period of 10–30 min. after infusion. The loss of the non-specific response appeared to be associated with an increase in the intensity of stimulus necessary to cause EEG arousal. This change in arousal threshold was caused, at least in part, by the action of the carrier agents. The infusion of up to 400 μg. of progesterone in a microcrystalline form in saline caused changes in the activity of many units in the anterior hypothalamus and adjacent regions; both excitatory and inhibitory effects were observed. However, every one of these prominent changes in unit activity was associated with a prolonged period of synchronized EEG. There was no evidence to suggest that progesterone was having any selective action on the hypothalamus.