Factors involved in blood oxygen transport were measured serially in the first, second and third trimester of pregnancy in 23 insulin-dependent diabetic women. Twenty-six non-pregnant diabetic patients served as a reference group. Diabetic pregnancy was associated with relative anemia, a significant increase in arterial pH, and hypocapnia. The concentration of red cell 2,3-diphosphoglycerate was significantly higher in the first trimester of diabetic pregnancy compared with non-pregnant diabetics (median value 16.4 vs. 15.0 mumol/g hemoglobin, p less than 0.02) and increased gradually from the first to the third trimester (16.4 to 17.2 mumol/g hemoglobin, p less than 0.01). The hemoglobin A1c concentration decreased simultaneously from 8.1% to 7.3% (p less than 0.01). The level of hemoglobin A1c in the first trimester was significantly lower than that in the non-pregnant diabetic patients (8.1 vs. 9.3%, p less than 0.01). In spite of the increase in red cell 2,3-diphosphoglycerate content and the decrease in hemoglobin A1c, factors known to reduce hemoglobin-oxygen affinity, the position of the oxyhemoglobin dissociation curve remained unchanged during diabetic pregnancy: P50 at actual pH in the first trimester, was 26.0 mmHg; in the second trimester, 26.9 mmHg, and in the third trimester, 26.8 mmHg (NS). These values of P50 at actual pH were identical with the value in the non-pregnant group (26.6 mmHg). Other factors influencing hemoglobin-oxygen affinity, such as hemoglobin concentration, hydrogen ion concentration and arterial oxygen saturation remained unchanged during diabetic pregnancy.(ABSTRACT TRUNCATED AT 250 WORDS)