Intracellular recordings have been obtained from forty-one preoptic area (POA) neurones at times up to 14 months after they were grafted into the third ventricle of the mouse. Thirty-one neurones were in grafts from hypogonadal (hpg) mice in which a reversal of the hypogonadism was seen (responders), six were in grafts from hpg mice in which no such reversal occurred (non-responders) and four were in grafts from normal mice. 2. The grafted neurones had a mean resting potential (Em) of -57 mV, a mean apparent input resistance (Rm) of 136 M omega and a mean membrane time constant (tau m) of 7.7 ms. The slopes of the current-voltage (I-V) relations were linear. Approximately a quarter of neurones in responders fired action potentials spontaneously either singly or in bursts. Such activity could underlie the release of gonadotrophin hormone-releasing hormone (GnRH) which is known to occur from such grafts. 3. Two types of response were seen when these neurones were depolarized to firing threshold from Em, in one group a single action potential was discharged; in the other group one or more action potentials arising from a transient, slowly rising and falling depolarization (low-threshold response, LTR) was recorded. Some cells in the former category exhibited a LTR when depolarized from a potential more negative than Em. 4. The commonest response to stimulation of the median eminence in responders was an EPSP either alone or in combination with an IPSP. Antidromic action potentials were seen in four neurones and in two of these cells excitatory synaptic inputs could be demonstrated when the host hypothalamus adjacent to the graft was stimulated. It is suggested that these responses may represent activation of an afferent input from the host to neurones in the graft. 5. The morphology of neurones in POA grafts was determined by intrasomatic injection of horseradish peroxidase (HRP). A variety of profiles were seen and although some neurones extended over distances of up to 635 microns and branched extensively only one appeared to enter the host tissue at the ventrolateral edge of the graft. 6. A comparison was made between grafted POA neurones and cells in the medial preoptic area (MPOA), a region which constituted a significant component of the grafted tissue. No significant difference was noted between neurones in the graft and neurones in the MPOA in terms of their passive membrane properties. With regard to the active properties MPOA neurones could also be classified according to whether or not a LTR was elicited when the neurone was depolarized from Em. The major difference between the grafted neurones and those in the MPOA lay in the proportion of cells which exhibited a LTR under such conditions, being significantly greater in the latter group.
