In young healthy individuals, an i.v. glucose bolus leads to an immediate increase in plasma insulin, whereas in non-insulin-dependent diabetic patients this early response is diminished, lacking or even negative. In the present study, we sought to determine whether negative responses were also present during square-wave glucose stimulation (transition from 18 to 25 mM), whether they represented a decrease in beta-cell secretion, whether they were accompanied by an altered response to arginine (5 g l-arginine bolus), and whether they were a consequence of ageing rather than of diabetes. A group of 12 patients (aged 53±2 years, mean±SE) with non-insulin-dependent diabetes (D) and 12 matched healthy controls (C; aged 47±1 years) were evaluated twice at an interval of 3 months. Other baseline values were body mass index (BMI) 28±1 (D) and 26±1 (C) kg/m², fasting C-peptide 0.85±0.12 (D) and 0.92±0.10(C) nmol/l, and fasting P-glucose 12.3±0.9 (D) and 5.8±0.1 (C) mM, P<0.05. Paradoxical responses (a decrease of two or more times the SD of the analysis within 15 min of increasing the glucose concentration) were seen in five diabetic patients for insulin (22±8%) and in nine diabetic patients for C-peptide (13±3%), but never in the healthy controls. Plasma glucose increased and protein decreased similarly, whether the responses were paradoxical or not. Paradoxial responses were reproduced after three months. Responses to arginine did not correlate with responses to glucose. In summary, in contrast to healthy matched controls, 40-75% of non-insulin-dependent diabetics show a marked initial decrease in beta-cell secretion upon square-wave glucose stimulation. This is probably specific to glucose stimulation, as it did not occur in response to arginine stimulation.