By means of vitrectomy in vivo investigations of the pathologic changes of the vitreous body including histology and chemical analysis have become feasible. The electron microscopical examinations of the tissue particles gained by means of vitrectomy displayed collagen fibrils which were partly irregular, most of the mesh-work being split up. The periodicity of these fibrils was about 64 nm, similar changes had been observed in human vitreous following perforating injuries, as well as in rabbit vitreous after blood injections. All typical stages of the lysis of red cells and their phagocytosis by large macrophages could be demonstrated. In order to get an accurate chemical analysis of the material resulting from vitrectomy it was first to all necessary to find out a method which enabled us to compare the concentrations of different chemical substances in the material gained by vitrectomy with the content of these substances in the whole vitreous body. Of 16 pairs of human post mortem eyes, enucleated between 3 and 5 hours after death, one eye was submitted to vitrectomy under constant suction- and infusion-pressure and cutting rate, the duration of surgery being different (3-9 min.); in each fellow-eye the whole vitreous body was removed. The comparison of the different concentrations of chemical substances showed a typical correlation. This method was performed in 13 clinical cases suffering from different kinds of vitreal changes. The concentration of protein was found to have increased in cases suffering from intravitreal hemorrhages, in cases suffering from shrinkage of the vitreous body after spontaneous complete resorption of intravitreal bleedings and in one case suffering from intravitreal membranes after endophthalmitis; in cases suffering from subvitreal bleedings the protein content of the vitreous body was approximately normal. The pathology leakage of the retinal vessels as it can be found in cases suffering from proliferative diabetic retinopathy caused no significant increase of the intravitreal protein concentration. The concentrations of lactic acid and glycogen were examined in order to find out if anaerobic glycolysis could be proved in the vitreous body of eyes suffering from ischaemic retinopathy. Only one case showed an increased concentration of lactic acid, in 3 cases there was a reduced content of glycogen. In these 3 cases the possibility of the existence of anaerobic glycolysis could be excluded, because in rabbits a rapid excretion rate of exceeding concentrations of lactic acid via retinal vessels had been found.