Recent chemical stimulation, ablation and electrophysiological recording experiments have suggested that an important osmosensitive region exists in the lateral preoptic area of the rat. The purpose of the present study was to examine the activity of single neurons in the lateral preoptic region of the rat brain in response to hypertonic, hypotonic and isotonic stimulation. Extracellular spontaneous unit discharges were recorded from 54 neurons in the lateral preoptic area of 27 hooded rats using glass micropipettes and standard recording techniques. Although frequencies of most units increased following the infusion of one or more solutions, a substantial number of units showed a decrease, while others did not appear to change. In general, the direction of change in frequency was constant for modulated responses from any given unit. The variability of the responses observed cannot be easily interpreted considering osmometric factors alone. Both osmometric and volumetric factors might be implicated in the changes observed in the activity of lateral preoptic neurons.