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J. tulc and J. vihovec

1. The possible role of active transport of Na in generating the transplacental potential difference (p.d.) was studied in the guinea-pig placenta perfused in situ by testing the effects of strophanthin or KCN added to the perfusate or of Na-free perfusion fluid.

2. The p.d. across the perfused placenta was about -20 mV, and was relatively steady during perfusion.

3. Strophanthin (10^-5 M) slightly increased the p.d. In a concentration of 2 x 10^-6 M strophanthin was without significant effect.

4. KCN (10^-3 M) decreased the p.d. When KCN was removed after a 30 min treatment the p.d. increased significantly exceeding the initial level. KCN (10^-4 M) slightly increased the p.d.

5. Perfusion with Na-free fluid decreased the p.d. to a steady level of about 50-60% of the control values.

6. Perfusion with Na-free fluid containing KCN (10^-3 M) for 30 min decreased the p.d. However, the effect did not seem different from the effect of KCN (10^-3 M) or Na-free fluid alone. When KCN was removed the p.d. significantly increased in spite of continued perfusion with Na-free fluid.

7. Excepting the effect of KCN (10^-4 M), which could not be explained, the observations were found to be consistent with active transport of Na from foetal to maternal side of the placenta as a source of the p.d. The Na transport could account for about 40-50% of the p.d.