Novel measures of coding based on inter-spike intervals were used to characterise the responses of supraoptic cells to osmotic stimulation. Infusion of hypertonic NaCl in vivo increased the firing rate of continuous (putative oxytocin) cells (Wilcoxon's z=3.84,P=0.001) and phasic (putative vasopressin) cells (z=2.14,P=0.032). The irregularity of activity, quantified by the log interval entropy, was decreased for continuous (Student's t=3.06,P=0.003) but not phasic cells (t=1.34,P=0.181). For continuous cells, the increase in frequency and decrease in entropy was significantly greater (t=2.61,P=0.036 and t=3.06,P=0.007) than for phasic cells. Spike patterning, quantified using the mutual information between intervals, was decreased for phasic (z=-2.64,P=0.008) but not continuous cells (z=-1.14,P=0.256). Although continuous cells showed similar osmotic responses to mannitol infusion, phasic cells showed differences: spike frequency decreased (z=-3.70,P<0.001) and entropy increased (t=-3.41,P<0.001). Considering both cell types together, osmotic stimulation in vitro using 40 mM NaCl had little effect on firing rate (z=-0.319,P=0.750), but increased both entropy (t=2.75,P=0.010) and mutual information (z=-2.73,P=0.006) in contrast to the decreases (t=2.92,P=0.004 and z=-2.40,P=0.017) seen in vivo. Responses to less severe osmotic stimulation with NaCl or mannitol were not significant. Potassium-induced depolarisation in vitro increased firing rate (r=0.195,P=0.034) but the correlation with decreased entropy was not significant (r=-0.097,P=0.412). Intracellular recordings showed a small depolarisation and decrease in input resistance during osmotic stimulation with NaCl or mannitol, and membrane depolarisation following addition of potassium. Differences in responses of oxytocin and vasopressin cells in vivo suggest differences in the balance between the synaptic and membrane properties involved in coding their osmotic responses. The osmotic responses in vivo constrasted with those seen in vitro, which suggests that, in vivo, they depend on extrinsic circuitry. Differences in responses to osmolality and direct depolarisation in vitro indicate that the mechanism of osmoresponsiveness within a physiological range is unlikely to be fully explained by depolarisation.