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Changes in brain activity after electrochemical stimulation of the preoptic area of prooestrous rats were studied by the measurement of the electro-encephalogram (EEG) of the frontal cortex and the recording of single neurones in the anterior hypothalamus. All rats were anaesthetized with urethane between 10.00 and 12.00 h to allow prolonged electrophysiological recording and to block the spontaneous surge of LH during the afternoon. Electrochemical stimulation was applied, between 12.00 and 14.00 h, as an anodal current through an implanted steel electrode; this caused the electrolytic deposition of iron and evoked the release of LH and ovulation.

Electrochemical stimulation of the preoptic area changed the cortical EEG, either immediately or after a delay of a few minutes, from a labile pattern with alternate periods of arousal and slow-wave sleep, to a stage of continuous arousal which persisted for the remainder of the recording period (2–3 h). Conversely, the EEG pattern of the cortex was not disturbed by electrolytic lesions placed in the preoptic area through a platinum electrode. Electrochemical stimulation of the arcuate region of the hypothalamus, the lateral septal area, the medial amygdaloid complex and the anterior parts of the thalamus caused no obvious change in the EEG patterns.

Ipsilateral anterior hypothalamic neurones, about 1 mm caudal to the focus of electrochemical stimulation, displayed an immediate decrease in electrical activity after application of the current. After 10–20 min however, the rates of discharge of action potentials in 9 out of the 16 neurones under consideration increased progressively from 0·5 to 15–25 action potentials/s and these rates were maintained until the recordings were lost after 90–230 min. No such acceleration in electrical activity was observed in neurones on the contralateral side.

Iron deposited during electrochemical stimulation was precipitated as sulphide and stained by Timm's method. There was a central damaged area of radius 0·6 mm surrounded by an 'undamaged' area with considerable infiltration of iron, up to a distance of 1·7 mm from the electrode tip. Cells within the area of infiltration did not stain for iron 10 min after electrochemical stimulation, but after 30 min, neural elements in this peripheral zone were stained in a manner similar to the Golgi method.

The concentrations of LH in the plasma remained unchanged in all rats for 10–15 min after electrochemical stimulation. Thereafter, the concentrations increased progressively and approximately in parallel to the changes in action potential activity until, after 2 h, the individual concentrations of 300–600 ng LH/ml were more than six times the values obtained before stimulation. Bilateral electrochemical stimulation resulted in appreciably higher concentrations of LH and produced values close to those observed during the pro-oestrous surge of the hormone.

Electrochemical stimulation during the afternoon of the day before pro-oestrus consistently advanced ovulation; this response occurred irrespective of whether the resultant lesions were large or small. The production of a large lesion during the afternoon of the day before pro-oestrus without concomitant deposition of iron by the use of a platinum electrode appeared to block the surge of LH on the subsequent day and the preovulatory follicles became atretic.

These results suggest that the ovulatory response to electrochemical stimulation is related primarily to an increase in the electrical activity of the hypothalamus and not to the destruction of brain tissue.