A 10-ml plasma sample was taken from each patient and analysed for urea, electrolyte, and carbamazepine concentrations, the latter being measured by gas chromatography, methaqualone being used as an internal standard. Plasma osmolality was measured by means of the depression of freezing point technique. The results were analysed with Wilcoxon's sum of ranks test. Details of the 16 patients are given in the table. Five patients were hyponatraemic. All were receiving carbamazepine in a daily dose exceeding 700 mg, and all had steady-state carbamazepine blood concentrations exceeding 35 ,tmol/l (8-3 Hg/ml). There was a strong statistical association between hyponatraemia and daily doses of carbamazepine over 700 mg (P= 0025) and between hyponatraemia and steady-state carbamazepine blood concentrations exceeding 35,tmol/I (P= 0005). The mean plasma sodium concentration in patients receiving more than 700 mg carbamazepine daily was 132+SE of mean 2-0mmol (mEq)/l, compared with 137±0-7 mmol/l in patients receiving smaller doses. Comparable figures for plasma osmolality were 266 ± 6-0 mmol (mOsm)/kg and 285 +2-3 mmol/kg respectively. Mean plasma sodium concentrations in those with steady-state