Factors affecting the evaluation of oral glucose tolerance (OGT) test have been studied. The test was performed on 95 women pregnant at 30-32 weeks of gestation and to 112 control subjects (68 female and 44 male). Pregnant and non-pregnant subjects were further divided into two subgroups according to normal or abnormal response in glucose tolerance test. Venous (vB) and capillary (cB) blood specimens were collected simultaneously for blood glucose determinations. In one series the o-toluidine method and in another the enzymatic glucose dehydrogenase method were used. Inter-relations of capillary and venous blood glucose levels were examined with respect to gestation, glucose tolerance and analytical method. Mean venous blood glucose was higher than capillary blood glucose in fasting blood (0-sample) for all subjects, when o-toluidine (o-tol) method was used, but with glucose dehydrogenase (GDH) no significant capillary-venous difference was seen. At blood glucose concentrations after glucose intake (1 and 2 h) mean capillary glucose was higher in all subgroups when measured by GDH, but by o-tol method only in the non-pregnant subjects with normal OGT response. In pregnant subjects with normal or abnormal OGT response, and in non-pregnant ones with abnormal response, mean venous glucose was higher or of the same order as capillary glucose. These findings emphasize the need of standardization of the sampling procedures and of the analytical methods used for measurement of blood glucose concentration as well as demonstrate the possibilities of misinterpretation of OGT test results.