The pH, PO2, PCO2, and lactate concentration were measured in umbilical blood samples obtained by cordocentesis from 208 appropriate-for-gestational-age and 196 small-for-gestational-age fetuses at 18 to 38 weeks' gestation. In the appropriate-to-gestational-age fetuses umbilical venous (n = 173) and arterial (n = 35) PO2 and pH decreased, and umbilical venous and arterial PCO2 increased with advancing gestation. Blood lactate concentration did not change. Compared with the appropriate-for-gestational-age fetuses, the small-for-gestational-age fetuses were hypoxemic, hypercapnic, hyperlacticemic, and acidotic. There was mixed respiratory and metabolic acidosis, and the decrease in pH was significantly correlated with both hypercapnia and hyperlacticemia. In the umbilical artery (n = 53) there was a significant linear correlation between the degree of hypoxemia and the degrees of hypercapnia or acidosis. In the umbilical vein (n = 143), the correlations between the degree of hypoxemia and the degrees of hypercapnia, hyperlacticemia, or acidosis were exponential.