Since the mid-seventies, a continual reduction of the European brown hare population has been observed in Poland and other European countries. Probably many intercorrelating factors play a role in this continous reduction. On the one hand, one has to take into consideration the anthropogenetic influences such as intensive agricultural cultivation, environmental pollution, loss of habitat, increased road traffic and hunting as probable reasons for the decrease in population. On the other hand, predators, climatic influences, cyclic fluctuations in population as well as diseases, in particular pseudotuberculosis, pasteurellosis, coccidiosis and European brown hare syndrome, are being discussed as negative influences on the evolution of the hare population. In cooperation with Prof. Z. Pielowski (Czempin Poland) 125 brown hares were shot in Czempin in 1993/94. Pathoanatomical, histopathological, virological, bacteriological, parasitological, reproductionbiological, electronmicroscopical and toxicological investigations were conducted to determine the significance of infectious diseases, parasites and harmful substances as factors of influence on the development of the brown hare population in Czempin. The present study could not ascertain a single cause for the decline of the population. Rather it showed that the decline must be attributed to multifactoral occurrences. The investigations showed that the most significant probable cause for the decrease on the one hand is the European brown hare syndrome (EBHS): using an ELISA testkit six (7,6%) samples were antigen-positive for EBHS virus (EBHSV). In one of these samples caliciviruses could be observed by negative staining electron microscopy. EBHSV antibodies were present in 38 (38%) of the hares. Pathohistological investigations yielded the following results: in 50% of the EBHSV antigen-positive and in 29% of the seropositive hares predominantly periportal hepatitis in the form of small scattered foci of mononuclear cells was found. These results indicate that in all probability EBHS in brown hares in Czempin had a subacute or chronic course of the infection. On the other hand the comparative high level of contamination with cadmium could be equally important to the dynamics of the hare population. In each of the 48 investigated kidneys cadmium was found in concentrations of up to 17,3 mg/kg FS (fresh substance). Brown hares with a cadmium contamination above average (4,4 to 17,3 mg/kg FS) showed a significantly higher incidence of tubulonephrosis than animals with a residue content of cadmium below 4,4 mg/kg FS. Although these alterations in the kidneys could have resulted from other causes, these findings suggest that chronic damage of the kidneys could have resulted from the cadmium residue content. Further possible factors influencing the development of the hare population were found in the analysis and search for residue contents of organochlorine compounds in 49 liver samples. In some of the cases it appeared that the detected maximum levels of endrin (0,13 mg/kg FS (fatty substance)), dieldrin (0,19 mg/kg FS) and heptachlor-epoxid (0,61 mg/kg FS) could have impaired the health of the affected individuals. It may be critical that some animals were simultaneously high contaminated with HCH, PCBs, heptachlor-epoxid and partly with cadmium, dieldrin and endrin. However the histopathological examination did not reveal any findings wich could definitely be brought into connection with a poisoning by organochlorine compounds. Furthermore, the comparatively high incidence of histological alterations found in the hares (especially interstitial pneumonia, haemosiderosis of spleen and liver, lymphoplasmocytic enteritis and interstitial nephritis) point to a reduced state of health. The present study was not able to substantiate the cause for the alterations observed. In addition, a reduced reproductive capacity in the male hares could be a further reason for the decline of the population. In 35 male hares the motility of epididymal sperms during breeding season (January to September) was investigated. The motility varied between 0% and 90%. However, in one- third of the hares the observed motility was below 10%, wich points to hypokinospermie. The results of the bacteriological and parasitological examinations lead to the interpretation that the typical harediseases (pseudotuberculosis, pasteurellosis, coccidiosis) as well as the contamination with lead and mercury and most of the organochlorine compounds (for example DDT) play a subordinate or insignificant role in the decrease of the harepopulation in Czempin.