The aim of the study was to determine the time required for plasma fructosamine concentration to increase after the onset of hyperglycaemia and decrease after resolution of hyperglycaemia. Healthy cats ( n=14) were infused to maintain either moderate hyperglycaemia ( n=5) (actual mean glucose 17 mmol/l) or marked hyperglycaemia ( n=9) (actual 29 mmol/l) for 42 days. Fructosamine exceeded the upper limit of the reference range (331 μmol/l) after 3–5 days of marked hyperglycaemia, took 20 days to plateau and, after cessation of infusion, took 5 days to return to baseline. Fructosamine concentration for moderate hyperglycaemia took longer to exceed the reference range (7 days, range 4–14 days), and fewer days to plateau (8 days) and return to baseline (1 day). In cats with moderate hyperglycaemia, fructosamine concentration mostly fluctuated under the upper limit of the reference range. The range of fructosamine concentrations associated with a given glucose concentration was wide. The critical difference for fructosamine was 33 μmol/l.