To investigate the interindividual variation in the absorption of sulphonylureas and its relation to unexpected hypoglycaemia during therapy with these drugs, serum concentration profile of glibenclamide, as well as plasma glucose and insulin response after an oral intake of glibenclamide were studied in 17 patients with non-insulin-dependent diabetes mellitus. In 12 patients glibenclamide was rapidly absorbed, reaching a peak concentration of 138.0 ± 15.8 ng/ml (mean ± sem) at about 2 h. However, in 5 patients the absorption of glibenclamide was delayed and reached a serum peak of 134.1 ±29.5 ng/ml at later than 4 h. This delayed absorption was reproducible. There were no significant differences in age, duration of diabetes, per cent of ideal body weight, fasting plasma glucose or HbA 1C between the rapid absorption group and the delayed group. However, autonomic nerve function, assessed by coefficient of variation of R-R intervals, was significantly impaired in the latter group in comparison with the former. Plasma glucose and insulin response to glibenclamide was also delayed in the delayed absorption group.