The transport of 131I-labelled polymer microspheres through the female reproductive tract of the rabbit has already been reported (Glover & Patterson, 1963; Glover & Wood, 1964), but the systemic distribution of residual unbound radioactive iodide after the process of labelling the microspheres has not been described in detail. This information is needed, however, in order to distinguish radioactivity in the uterus which is bound to the microspheres from that which arises from unbound radioactive iodide. The level of radioactivity found in the uterus after administration of suitable quantities of 131I as sodium iodide has therefore been determined. Twenty-one oestrous rabbits received 1–5 μc Na131I. Of these, ten were injected intravenously, and in 11 the iodide was deposited in the anterior vagina. The animals were killed by i.v. injection with sodium pentobarbitone, 3 gr./ml., (Euthatal, May and Baker Ltd.) after 3, 6 or 12 hr. and their