Feto-maternal vascular (PD(F-M)) and amniotic maternal (PD(A-M)) potential differences were measured simultaneously on seven occasions in six conscious pigs of 100-106 days gestation. Resting values of PD(F-M) and PD(A-M) were not significantly different although the range was wide. Fetal intravascular injection of 20 microg adrenaline, but not of saline, was associated with a prompt reversible change, of equal magnitude, in both PD(F-M) and PD(A-M). In some experiments polarity was reversed. Feto-amniotic potential difference did not change. There was no change in fetal plasma K+ and Na+ concentrations. Because of the simultaneous and equal alterations in PD(F-M) and PD(A-M) following adrenaline and the anatomical configuration of the pig conceptus, we conclude that the catecholamine modifiable component of PD(F-M) is generated by electrogenesis in the pig placenta, probably by its chorionic (trophoblastic) cell layer.