Our previous short term studies of the placental transfer of cimetidine in the pregnant sheep showed that following infusion of cimetidine to the mother, there was a striking (greater than 25 to 1) maternal to fetal gradient at apparent steady state. The present study examines two possible explanations: 1. That the gradient reflects very low placental permeability to cimetidine, such that in short-term studies a true steady state is not achieved. 2. That the gradient results at least in part from differences in protein binding in maternal and fetal plasma. To determine whether the gradient was due to very low placental permeability to cimetidine, pregnant sheep were infused with cimetidine continuously for 6 days (n=5). The large 25 to 1 cimetidine gradient did not diminish throughout this period, thereby ruling out a non-specific "barrier" effect of the placenta. In vitro protein binding studies of cimetidine in maternal and fetal plasma showed low binding (range = 16 to 46%) and minimal differences between mother and fetus. We conclude that the observed maternal to fetal cimetidine gradient cannot be explained by either low placental permeability, or by differential protein binding of cimetidine in maternal and fetal plasma.