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1. Extracellular action potentials recorded from 798 neurones in the mediobasal hypothalamus (MBH) of pentobarbitone anaesthetized male rats were analysed for a change in excitability following stimulation in the medial preoptic and anterior hypothalamic areas. 2. An increase in excitability characteristic of orthodromic excitation was observed from 11-5% (n=92) of MBH neurones. Latencies for excitation were shorter for cells tested with anterior hypothalamic area stimulation (n=42; mean 5-4 +/- 2-6 msec S.D.) than for cells tested with medial preoptic stimulation (n=50; mean 15-2 +/- 7-2 msec S.D.). With spontaneously active neurones, excitation was followed by a decrease in excitability lasting 150-250 msec. An initial decrease in excitability, suggestive of post-synaptic inhibition, over a wide latency range (4-30 msec) and with duration of 100-400 msec was observed from 3-6% of MBH neurones. 3. Features of antidromic invasion were observed from 149 MBH neurones. From the medial preoptic area, the latency range was 0-5-38 msec (mean 7-8 +/- 5-5); from the anterior hypothalamic area the latency range was 0-4-9-5 msec (mean 3-1 +/- 2-3). Occasionally an abrupt decrease in latency followed an increase in stimulus intensity. Most cells followed paired stimuli at frequencies up to 500 Hz. Axon conduction velocities were estimated to be under 2-0 m/sec. Antidromic invasion was usually followed by a decrease in excitability lasting approximately 100-150 msec. 4. Twenty MBH neurons displayed antidromic invasion from both the medial preoptic or anterio hypothalamic areas and one other stimulation site: the median eminence (five cells); the amygdala (six cells); the region of thalamic nucleus medialis dorsalis (three cells) and the midbrain periaqueductal gray (six cells). Interaction studies indicated that the axons of these cells branched close to the origin of the axon itself. 5. Antidromic invasion from the surface of the median eminence identified thirty-nine tuberoinfundibular neurones. Stimulation in the medial preoptic and anterior hypothalamic area produced orthodromic excitatory (n = 5) and inhibitory (n = 4) actions on HVM neurones, but was without an action on most other neurones (n = 30). Tuberoinfundibular neurones in the ventromedial nucleus also responded to stimulation in the amygdala, but usually at latencies greater than that for medial preoptic area evoked responses. 6. These observations indicate a close relationship between MBH neurones and cells located in both the amygdala and the medial preoptic-anterior hypothalamic area. The data for tuberoinfundibular neurones indicates that several extrahypothalamic areas may send fibres to these cells. These pathways may be important for the adaptive neuroendocrine responses reported in the literature.