Amniotic fluid (AF) and arterial blood were obtained almost simultaneously from 59 healthy third-trimester pregnant women. Oxygen and carbon dioxide tensions (Po2Pco2), pH, bicarbonate (Bicarb), total carbon dioxide (TCO2), Iactate (L), and pyruvate (P) concentrations were measured. Only AF-Bicarb, AF-TCO2, AF-pH, and AF-P showed a statistically significant correlation, negative, with gestational age. AF-Po2, AF-pH, AF-Bicarb, and AF-TCO2, were lower, and AF-Pco2was higher than in maternal blood. Amniotic fluid lactate was about 6 times, and AF-P was 2 times higher than maternal blood levels. The latter, the low bicarbonate, and the high Pco2all contributed to the acidic pH of AF. Amniotic fluid pH was affected by fetal and maternal variables, notably maternal pH. There was no correlation between AF-Po2and Pao2(arterial oxygen tension), between AF-Pco2and Paco2(arterial carbon dioxide tension), or between AF-L and blood L AF-Po2, AF-Pco2, and AF-L may correlate better with the intrauterine fetal status than AF-pH. Normal values obtained in this study should be useful as a baseline to monitor changes in the intrauterine environment in abnormal pregnancies.