Potential differences between mother and foetus at different gestational ages in the rat, rabbit and guinea-pig.

1. Potential differences associated with fluid compartments of rat, rabbit and guinea-pig conceptuses have been measured. [Na(+)] and [Cl(-)] in maternal plasma and amniotic fluid from these three species were also determined.2. Transplacental potential differences of about 15 mV (foetus positive) were found in the rat, of approximately 0 mV in the rabbit, and of about 18 mV (foetus negative) in the guinea-pig.3. Amniotic fluid potential differences appeared to arise indirectly from the transplacental potential difference in the rat, from the foetal gastric mucosa in the rabbit, and possibly from the foetal gastric mucosa and indirectly from the placenta in the guinea-pig.4. The results are discussed in the context of Na(+) transfer to the foetus, and on this basis tend to question the general assumption that almost all Na(+) reaching the foetus passes across the placenta.