To determine the placental clearance of lactate and bicarbonate in sheep, the fetal side of an isolated cotyledon and the umbilical circulation of the total placenta were artificially perfused. The release and uptake of lactate and bicarbonate by the perfusion fluid and the fetomaternal concentration differences of these substances were measured. From these data, the clearance of lactate and bicarbonate was determined to be 0.9 (SE = 0.2) ml/h/g of placental tissue. The production of lactate by the placenta was calculated to be about 30 µmol/min, the placental permeability of lactate was evaluated to be about 1.3 ml/h/g of placental tissue. These results indicate that fetal concentration changes of lactate and bicarbonate due to placental transfer occur with a half time of about 4 h. It is concluded that the lactate and bicarbonate permeability of the syndesmochorial placenta of the sheep is about 20 times smaller than the placental permeability of the hemochorial placenta of the guinea pig. It seems unjustified to draw any conclusions from experimental data obtained in the sheep placenta for the transplacental acid-base balance between mother and fetus in human beings.

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From the research work of Lemtis (4) and Künzel (3) we know that uterine blood flow (UBF) decreases following vena caval occlusion (VCO) when the mean arterial blood pressure (BP) decreases and/or the venous pressure increases. Fetal heart rate (FHR) will drop when the UBF falls below a certain critical level and this decrease in FHR is associated with a.fall in Po 2 and oxygen Saturation which initially provokes a pressor response in the fetus. Until recently i t has only been possible to consider such pathophysiological questions in animal models, but with the development of the tcPo 2 -technique (1) one is now able to investigate this phenomenon in human labour. MATERIAL AND METHODSIn 43 cases FHR, fetal tcPo 2 , fetal skin blood flpw and maternal uterine activity were continuously monitored and analysed in mothers^ who were \intermittently in supine Position. The observations were all made in the late first and second stage of labour. RESULTS The mean duration of the period spent in supine position was 6 min. In 39 of the 43 cases fetal tcPo 2 was found to decrease when the mother adopted the supine position (91%). During this time uterine activity remained constant and a classical VCO-syndrome followed in only two cases (4.65%). Fetal skin blood flow äs measured by the heating current increased in 60% and remained constant in 40% of cases. FHR showed three different patterns: no change (44%), acceleration (40%) or deceleration (16%). The tcPo 2 level before adoption of the supine position showed no significant difference for these three types of FHR pattern. There was however a significant difference in the initial FHR depending on wether the response was an acceleration or deceleration (130 and 132 bpm) compared with the group where .the FHR (137 bpm) remained constant (2-«0.025; 2-*<0.05). A significant difference was found between the recovery time of tcPo 2 (after returning to the lateral position) of the group with no FHR response and the group which showed an acceleration or decleration (2