Physiological changes in carbohydrate tolerance were studied between the beginning and end of pregnancy. Amongst 145 oral glucose tolerance tests performed between the 9th and 40th weeks, results indicated that carbohydrate tolerance evolved throughout pregnancy. The first 24 weeks were characterised by a change in the shape of the glucose tolerance curve, in the form of horizontalisation of the terminal part, but with no increase in early blood glucose figures. It was only after the 24th week that mean blood glucose levels were seen to be increased. The interpretation of glucose tolerance tests during pregnancy should take these physiological changes into account. Critical values, above which diabetes must be suspected, are different at the beginning and end of pregnancy. The critical point would appear to be around the 24th week.