Glycosylated haemoglobins and glycosylated plasma proteins were measured by affinity chromatography using Glycogel B, in 2-40 weeks old lean (+/+) and Aston obese hyperglycaemic (ob/ob) mice, and compared with changes in glucose concentrations. Lean mice showed an increase in glycaemia at weaning (3 weeks), and a small rise thereafter. In comparison with lean mice, ob/ob mice showed decreased glucose concentrations at 2 weeks, euglycaemia at 3-6 weeks, and hyperglycaemia from 7 weeks. The increases in glycaemia were followed by elevations of glycosylated haemoglobins after 1 week in ob/ob mice and 2 weeks in lean mice. Glycosylated haemoglobins were significantly elevated in ob/ob mice at 8-40 weeks. The glycosylated haemoglobins and glucose concentrations were correlated (r = +0.7; p less than 0.01; n = 96) for all mice studied. There was no correlation between glycosylated plasma proteins and glucose, or between glycosylated plasma proteins and glycosylated haemoglobins. Glycosylated plasma proteins were unchanged with age in lean mice, and were not significantly raised in ob/ob mice. The results indicate that glycosylated plasma proteins are a poor indicator of the glycaemic environment in mice due to their rapid turnover. In contrast, measurement of glycosylated haemoglobins provides a convenient and reliable assessment of blood glucose homeostasis in the preceding 1-2 weeks.