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1. Extracellular action potentials were recorded from 299 single units in the preoptic and anterior hypothalamic areas of fifty-two female rats anaesthetized with urethane. The units were categorized by their response to single biphasic pulses (ca. 1 mA; 1 msec duration) applied to the ventromedial/arcuate region of the hypothalamus.

2. Experiments with five lactating rats demonstrated that the effective zone of stimulation was confined within the ventromedial/arcuate region. This observation was supported by further evidence obtained during unit recording sessions.

3. Antidromic action potentials were recorded from 122 (41%) of the neurones monitored in preoptic and anterior hypothalamic areas. These Type A cells were characterized by their very slow discharge rate (median for spontaneously active units 1·2 spikes/sec) when contrasted with adjacent cells (median 3·9 spikes/sec). Orthodromic action potentials were not observed in thirty-six Type A cells. The antidromically identified neurones had an average conduction rate of 0·32 m/sec. The absolute refractory period of the soma, computed by separation of the IS and SD components of the antidromic action potentials recorded from forty-four neurones, ranged from < 3·0 to > 100 msec. With ten units it was not possible to obtain a soma dendritic (SD) wave by antidromic activation even though the initial segment (IS) wave was seen clearly and the orthodromic potentials consisted of both IS and SD waves.

4. Type B cells (32% of population) were excited and/or inhibited by the ventromedial/arcuate stimulation but were not antidromically activated. This post-stimulatory change in discharge rate lasted for up to 400 msec.

5. Type C cells (27% of population) showed no change in spontaneous activity after stimulation of the ventromedial arcuate area. These neurones had discharge rates similar to Type B cells.

6. The previously reported prooestrous increase in firing rate, recorded from neurones in the ventral part of the preoptic/anterior hypothalamic area was restricted to cell Types B and C.