There is still controversy concerning the reference ranges for glucose tolerance tests in pregnancy. The WHO has recommended the universal use of the 75 g oral glucose load with 2-h post-load values of greater than 6.7 mmol l-1 to be considered impaired glucose tolerance (IGT) in the non-pregnant, and equivalent to gestational diabetes in the pregnant. Some data are available for pregnant Caucasians but little information is available for other ethnic groups. Oral glucose tolerance tests (75 g) have therefore been performed in 189 pregnant women in rural Tanzania. Mean fasting blood glucose values were 4.0 mmol l-1 in non-pregnant women, and 3.7, 3.5, and 3.3 mmol l-1 in pregnant women in the first, second, and third trimesters, respectively. Two-hour OGTT values were 4.7 mmol l-1, and 4.6, 4.5, and 4.2 mmol l-1 while the upper limit of normal values (mean + 2SD) were 7.1 mmol l-1, and 6.8, 6.8, and 6.1 mmol l-1. The 2-h glucose levels are therefore close to WHO recommendations but lower than those reported for Caucasians. By contrast with reports for Caucasians, glucose tolerance did not deteriorate during pregnancy. The prevalence of diabetes and IGT was zero in the pregnant group.