Maternal and umbilical vein digoxin concentrations were determined in 16 mothers and fetuses with severe Rhesus-D disease, eight with, and eight without prior digitalization of the mother, when umbilical cord puncture was performed for diagnosis and intrauterine blood transfusion. In the eight patients without digoxin treatment, the digoxin concentrations in both the mother and the umbilical vein were below the limit of detection (less than 0.3 nmol/l). In the other eight patients digitalization of the mother was started 24-48 hours before the first umbilical cord puncture. The maternal and umbilical vein digoxin concentrations were determined on 26 occasions. Except for two instances, digoxin concentrations in the umbilical vein were always below 1 nmol/l. The mean ratio of maternal to fetal digoxin concentrations before initial transfusion was 2.51 ( +/- ISD = 1.47) and before later transfusions 1.67 ( +/- ISD = 0.61). The differences in mean ratios between initial and later transfusions are not significant (p = 0.16). The mean ratio for the total group was 1.93 ( +/- ISD = 1.01). There was no correlation between the maternal to umbilical vein digoxin ratio and either gestational age or umbilical venous hematocrit. The results of our study indicate that the therapeutic effect of transplacental digitalization in severe Rhesus disease is questionable and that a multicentre randomized trial would be necessary to evaluate whether this treatment is of benefit.