1. The responses of neurones in the anterior hypothalamic and preoptic areas (POA/AHA) to stimulation of the medial forebrain bundle (MFB) have been studied in urethane anaesthetized female rats. Extracellular unit recordings have been made from 150 neurones which were responsive to a single stimulus applied to the MFB at the level of the mammillary nucleus. 2. Forty-five per cent of these cells were orthodromically activated with latencies ranging from 7.5 to 100 msec. However, the majority of cells responded with latencies of less than 40 msec. 3. Marked inhibition of spontaneous activity was observed in 41.5% of the units. Response latencies of up to 40 msec were observed in these cells, the inhibitory periods lasting up to 150 msec. 4. A small proportion of cells (13.5%) were antidromically activated and the average conduction velocity of these neurones in the POA/AHA with axons passing down to the mid-brain was estimated to be 0.3 m sec-1. It is suggested that they represent part of the descending MFB. 5. The experiments did not show any discrete topographical organization of cells in the POA/AHA which could be driven by MFB stimulation although the units tended to be located in more lateral rather than medial areas. 6. The responses to iontophoretically applied dopamine (DA) or noradrenaline (NA) was tested on sixty-one cells. These amines suppressed the activity of the majority of both orthodromically activated and inhibited units; the remaining cells were unresponsive. 7. These results provide electrophysiological evidence for both a direct and indirect input of MFB fibres to cells in the POA/AHA and that these inputs can be either excitatory or inhibitory. The data also indicate that a small number of fibres in the descending MFB originate from cells in the POA/AHA. 8. The sensitivity of these units to NA and DA suggests an inhibitory aminergic input, although this evidence is as yet indirect. 9. These connexions of the MFB, with neurones in the POA/AHA may be part of the neural circuits important for extra-hypothalamic modulation of gonadotrophin secretion.