The placental transfer from maternal to fetal circulation of ritodrine, a tocolytic agent used in obstetrics and of a structurally related physiologic catecholamine,norepinephrine was studied in vitro using dual perfusion of isolated human placental lobules. The clearances of ritodrine and norepinephrine represent 34 and 24% respectively of tritiated water clearance taken as reference. The relatively low ritodrine clearance may be explained by its small molecular weight and hydrosoluble nature, but not that of norepinephrine. For the latter, a strong membrane limitation and efficient placental catabolism may be implicated. We conclude that some of the fetal effects of maternal infusion of these amines are the result of the method in which the drug is transferred.