Maternal arterial, umbilical venous, and umbilical arterial blood were obtained at fetoscopy in the second trimester and analyzed for blood gases and acid-base indexes. In comparing umbilical venous (N = 31) with maternal arterial (N = 39) samples, the mean (+/- SD) PO2 was lower (55 +/- 7 versus 100 +/- 15 mmHg, P less than .001) and PCO2 (37 +/- 4 versus 34 +/- 4 mmHg), bicarbonate (20 +/- 2.0 versus 18.5 +/- 1.5 mmol/L), lactate (1.10 +/- 0.25 versus 0.80 +/- 0.30 mmol/L), and base excess (-4.2 +/- 2.3 versus -6.0 +/- 1.5 mmol/L) were higher (P less than .001). The mean umbilical venous pH did not differ significantly from the maternal arterial (7.358 +/- 0.040 versus 7.373 +/- 0.035). Comparing umbilical arterial (N = 21) with umbilical venous (N = 31) samples, the PO2 level was lower (34 +/- 4 versus 55 +/- 7 mmHg, P less than .001) and PCO2 higher (42 +/- 4 verus 37 +/- 4 mmHg, (P less than .001); the pH was marginally lower (7.339 +/- 0.03 versus 7.358 +/- 0.040; P less than .05) and bicarbonate higher (22 +/- 1.5 versus 20 +/- 2.0 mmol/L, P less than .001). Umbilical arterial and umbilical venous values were similar in the case of lactate and base excess.