Appropriate insulin therapy may preserve or improve islet B-cell function whereas the effects of overinsulinisation are unclear. Pancreatic islet B-cell function was therefore studied after overinsulinisation of normal rats for 4 weeks (fed blood glucose 2.2-4.5 mmol/l, controls 4.1-7.0 mmol/l). Insulin secretion was assessed by a 3-h hyperglycaemic clamp (10.0 mmol/l) performed 1, 48, and 120 h after insulin withdrawal (n = 6 in each group). When the clamp was performed 1 h after insulin withdrawal, clamp insulin concentration was 1.6 +/- 0.1 micrograms/l, compared to 9.3 +/- 1.0 micrograms/l in control rats. The integrated area under the plasma insulin concentration curve was also significantly decreased (4.8 +/- 0.4 vs 20.3 +/- 2.2 micrograms.l-1.h-1, p less than 0.001), but recovered to 9.4 +/- 1.0 micrograms.l-1.h-1 after 48 h, and to 17.5 +/- 1.4 micrograms.l-1.h-1 after 120 h. Pancreatic insulin contents were decreased at 1 h (6 +/- 1 micrograms/g wet wt) and 48 h (54 +/- 12 micrograms/g wet wt) but not at 120 h (221 +/- 30 micrograms/g wet wt) after withdrawal (controls, 303 +/- 29 micrograms/g wet wt) and there was a strong relationship with pancreatic preproinsulin mRNA and the clamp insulin response. Thus, overinsulinisation with prolonged periods of low blood glucose concentrations impairs islet B-cell function, but is reversible over 5 days.