According to recent studies, glycosylated haemoglobin (HbA1) is subject to rapid variations. In order to verify this point, nine insulin-dependent diabetic patients underwent a 24-hour normalization of glycaemia with an artificial pancreas. There was a non-significant decrease of HbA1 from 9.26 +/- 1.03 p. cent to 8.69 +/- 0.84 p. cent (mean +/- s.d.). An explanation to the apparent discrepancies between the different studies is proposed. The variations observed could largely be due to the inclusion or non inclusion in the assay methods of a subfraction of glycosylated haemoglobin: a Schiff's base likely to undergo glysosylation in a rapidly reversible manner.