The maternofetal potential difference (PD) between catheters in maternal and fetal blood vessels has been measured in conscious sows between 97 and 107 days of gestation. The maternofetal PD was -18 +/- 4 mV (mean +/- SE, n = 13, fetus negative) on the day of surgery and -29 +/- 5 mV (n = 6) on the day after surgery. Injection of 2, 20, or 200 micrograms of epinephrine into the fetuses caused a marked rapid change in maternofetal PD such that the fetus became less negative and, in some cases, became positive with respect to the mother. The maximum change, obtained with 20 micrograms, was 19.9 +/- 5.6 mV (n = 7); measurements of fetal plasma epinephrine concentrations (using high-performance liquid chromatography) after injection of this dose gave a time 0-extrapolated concentration of 436.5 +/- 169.0 nmol/l (n = 4). Injection of 20 micrograms of the beta-agonist isoprenaline caused a maximum change in PD of 20 +/- 4 mV (n = 6); 2 mg of the alpha-agonist phenylephrine was required to produce a similar change (15 +/- 2 mV, n = 6). Injection of the beta-antagonist propranolol (1 mg) reduced the effect of 20 micrograms epinephrine by 40%. The effect of catecholamine on maternofetal PD is similar in polarity and specificity to that found for transplacental PD in vitro in the same species. There is, however, a difference between resting maternofetal and transplacental PD that remains unexplained.