Circulatory and respiratory reactions during the implantation of joint endoprostheses have been observed for years. The components of the bone cements used for anchoring (methyl methacrylate monomer) or the outpour of bone marrow substances from the affected bone marrow cavity are thought to be chiefly responsible for these effects. However, our previous investigations demonstrated unambiguously that reactions of this type can be attributed to direct nervous-reflex mechanisms triggered by the pressure rise in the medullary canal, and that, in addition, bone marrow embolism plays a part in serious circulatory incidents. The investigations on which the present paper is based have confirmed the mechanism of circulatory reactions triggered by direct nervous reflex and have shown once again that the release of cement monomers is not of any significance, at least not in animal experiments. In regard of the characteristics of these reflex processes, it has been possible to show by both surgical and pharmacologic interventions in the vegetative nervous system, performed before and during pressure experiments, that these reflex processes are caused not so much by an increase of the vagus tone as by a central inhibition of the sympathicus. Neither administrations of atropine nor bilateral cervical vagotomy are able to prevent the reactions. This was only possible under the influence of a ganglion blocker.