Heavy metal content was monitored in small mammals inhabiting mountain ecosystems and two industrial polluted regions in Bulgaria. Rodents (Microtus arvalis, M. rossiaemeridionalis, Clethrionomys glareolus, Pitymys subterraneus, Chionomys nivalis, Apodemus flavicollis, A. sylvaticus, and Mus macedonicus) were used as zoomonitors. Pathological changes in chromosome status, hematological indices, and blood cell morphology were analyzed in the context of heavy metal bioaccumulation. Significant correlations were obtained between the heavy metal load of zoomonitors and the frequency of chromosomal aberrations and pathological changes in erythrocytes (mainly micronuclei and basophilic granulations). It is suggested that mercury is a strong damaging factor for chromosomes and red blood cell apparatus.