Extracellular recordings were made from the neostriatum of rats anaesthetised with halothane. Normal male and female animals were used as well as castrated animals with implants containing oestradiol-17 beta (E2). In animals with high levels of circulating oestrogen, intact pro-oestrous rats and females and males bearing an E2 implant, individual units could be recorded that were spontaneously active and with axons that could be excited by stimulation in the crus cerebri. Such antidromically identified striato-nigral neurones were invariably silent in male animals and in ovariectomised females. In contrast to animals in pro-oestrus, intact female animals during metoestrus showed the 'male' pattern of striatal cell activity. These results show that E2 can stimulate spontaneous firing of striato-nigral neurons, and that this action of E2 is not sex-dependent.