We have carried out intravenous glucose tolerance tests with measurement of plasma glucose, insulin and C-peptide concentrations on 66 premenopausal and 92 postmenopausal non-obese caucasian women. After adjustment for the effects of a number of possible confounding variables, including age and body mass index, there was little difference between pre and postmenopausal women in glucose and insulin concentrations either fasting or in response to intravenous glucose. Mathematical modelling analysis of the resultant plasma concentration profiles was used to obtain measures of insulin sensitivity, secretion and elimination, and non-insulin dependent glucose disposal. We found reciprocal differences in mean insulin sensitivity (increased by 50%) and non-insulin dependent glucose disposal (decreased by 30%). Plasma C-peptide response and pancreatic insulin secretion were markedly lower in the postmenopausal group (-35% and -51% respectively). However, the rate constant for insulin elimination was also lower in these women. As a result, intravenous glucose tolerance test plasma insulin concentrations were not significantly different between the two groups. We conclude that, despite the occurrence of little or no variation in plasma glucose and insulin concentrations, the menopause is associated with significant changes in insulin metabolism.