With the purpose of studying the influence of polyvinylpyrrolidone (PVP) on the disposition of oxytetracycline (OTC), OTC was administered i.v. to male rats in two formulations, one of which contained 14C-labelled PVP. From the rats, whole-body sections (30 mum) were taken which were subjected to three analytical procedures: autoradiography to determine the distribution of the PVP-14C, fluorography and bioautography to obtain information on the OTC-distribution. The experimental results suggest equilibrium complex formation between OTC and PVP, it being mainly the complex which is i.v. injected when both products are combined in an injection formulation. Such complex formation can explain the lower toxicity of OTC when injected in combination with PVP as well as the slower build-up of OTC-levels in some organs, compared with that after OTC administration without PVP. Beyond this kinetic effect of PVP on the fate of OTC no evidence was obtained for an influence of PVP on the OTC distribution pattern per se, or on its ultimate antibiotic activity in the different organs. Some details of the OTC and especially of the PVP distribution are discussed.