Twelve biochemical values of the serum (concentrations of sodium, potassium, calcium, glucose, urea nitrogen, creatinine, total protein, and enzyme activities of ALP, CPK, AspAT, AlAT, amylase) were determined for 102 bisonBison bonasus (Linnaeus, 1758) coming from the free-ranging population of the Białowieża Forest and from the breeding enclosure. The animals were culled in the winter periods of 2000/2001 and 2002/2003. In adult males only the activity of AlAT is significantly higher than in adult females. The remaining values do not differ significantly between the sexes in any of the three age groups (I—calves, II—youngsters, III—adults). Also, no significant differences in serum parameters were observed between healthy males and the ones exhibiting symptoms ofposthitis/balanoposthitis. It was established that the following changes in the values of serum parameters are age-dependent: a decrease in potassium and glucose concentrations and the enzymatic activity of ALP, and an increase in AspAT activity. The results were compared to the corresponding data from the research carried out 20 years earlier (1980–1984) and it has been discovered that the values of seven parameters are significantly different as opposed to the previous results. The following differences were established: currently the concentrations of potassium and urea nitrogen are higher in all age groups; the concentration of glucose is higher in the groups of calves and youngsters; creatinine concentration is lower in adults; the activity of AspAT has increased in youngsters, only; the activity of AlAT has increased, and of amylase has decreased in all animals examined. The changes in the values of serum parameters were analysed in the context of data obtained from post-mortem examinations and we conclude that those changes reflect the growing number of pathomorphological changes in internal organs of the bison dissected. Changes localised mainly in the liver and lungs could be related to parasitical infestations; others were noted in kidneys and in the prepuce and penis area in the males withposthitis/balanoposthitis symptoms. In the discussion it is suggested that the deterioration of the physical condition of European bison inhabiting the Białowieża Forest may be connected to such environmental and populational factors as: winter supplementary feeding, winter aggregations, and a decline in immunity related to inbreeding.